I used to get free over-the-air channels; but now I don't. I have Comcast Internet connection but when I went to the local office they told me that I will need to pay monthly fees and box rental fees to get any channels.
You stated, "I used to get free over-the-air channels; but now I don't".
You stated you have a paid subscription to Comcast Internet. You did not state that you have a paid subscription to any level of Comcast Cable TV - Limited Basic Service or higher.
If you were receiving some "FREE" channels via cable TV, without paying any monthly subscription fees, that was a technical oversight that Comcast has now corrected. A technician has put a "filter" onto the settings for your account so you do not receive FREE cable TV service. Another possibility is that in your local area, Comcast has migrated to "all-digital" TV service and has then been able to encrypt what were previously the "Clear QAM" limited-basic channels.
Even with "free" over-the-air channels, Comcast is required to pay monthly per-subscriber "retransmission fees" to your local stations. Comcast provides a state-of-the-art master antenna system to provide reliable retransmission of your local TV stations. This is not a free service to the end user.
If you genuinely want FREE over-the-air channels, you will need to connect and configure an antenna. If you have a pre-2007 TV receiver, you might also need a Digital TV converter box to convert the ATSC digital OTA signals to the standard-definition NTSC format that your older TV can display.
Although you might get good reception using an indoor antenna for OTA channels, it's quite possible you may need an OUTDOOR antenna to obtain reliable signal quality on many of your local stations.
If you want any level of TV video service from Comcast, you will have to subscribe to, and PAY FOR, Limited Basic Cable sevice or a higher-tier level fo service.
Here's my understanding of what appears to have happened with regard to what you call "over the air" channels:
The basic channels from Comcast, channel 1 to channel 19 or so, used to be carried on the wire in analog. These are the channels that you refer to as "over the air". All other Comcast channels were (and are) digital.
Years ago, most older TVs would receive TV using an antenna (over-the-air, or OTA). At that time, OTA signals were analog and conformed to a standard called NTSC. Comcast used to put those same NTSC analog signals on their cable (though the carrier frequencies for these channels were slightly different from the carrier frequencies of the over-the-air signals). They carried these analog NTSC signals for years - until recently.
The older TVs had a setting to select either NTSC from an antenna or NTSC from cable (adjusting for the altered carrier frequencies). One could easily watch these analog channels from Comcast by simply setting the switch on the TV to "Cable" and plugging in the wire from Comcast into the back of the TV. No Comcast box required!
I'm guessing the Comcast began to feeling like they were losing out on money as long as they kept the analog signals on the wire. Why? I believe there are two answers:
1) Comcast logically reasoned that they could fit more digital channels on the wire if the content on the analog channels was instead carried digitally. Digital consumes less bandwidth than analog. (And of course, more channels mean more money for Comcast.)
2) If a customer does not pay his bill, Comcast can easily remove a customer's digital reception by flicking a switch at the Comcast office. ... Apparently, this denial of service is not so easy to accomplish for the analog signals. To deny access to the analog channels, Comcast must send a technician to the location of the customer who then must climb a pole and physically remove the connection.
I've known a couple of people whose Comcast accounts were terminated but their analog signals remained on the wire. These people continued to watch "basic" TV from Comcast ...until the Tech arrived and physically disconnected them.
Now Comcast is completing the removal of all NTSC analog signals. Today, customers who only want the very minimum service from Comcast (approximately $20/month) to receive what had been the analog channels must get the Digital Transport Adapter (DTA), or a full function Comcast digital cable box or they could use a QAM receiver which might be built into their TV.
Note: QAM will only allow for the reception of unencrypted digital cable channels. Digital channels can be sent “in the clear”, that is, unencrypted, or they can be encrypted. If a Comcast digital channel is encrypted, then only a Comcast box will receive and decode the channel.
Some people have speculated that Comcast will eventually encrypt all of the their digital channels. I don't know if that's true. I wouldn't know if they had done it already or not since I use only Comcast boxes in my house and I can't know if a channel I am receiving was decrypted by my Comcast cable box or not. But, if Comcast does eventually encrypt all their channels, then they will have total control over everyone's cable box and QAM receivers will be worthless.
One final note to everyone: Even if your TV does not have a QAM receiver in it, all modern TVs have a digital over-the-air (OTA) ATSC receiver built in. With a good antenna (preferably on top of your house or apartment building) you can receive spectacular high definition TV with 5.1 stero sound for absolutely free!
If you subscribe to both high speed internet and limited basic, then you are entitled to the no cost DTA digital adapter to receive limited basic channels.
If you have HSI without paying for limited basic, then you are out of luck until you add limited basic or some other digital TV package to your existing high speed internet subscription. If you have HSI only, then you should not have been getting any TV channels in the first place, and you were getting a free ride.