Lost ability to change WAN IP address by changing MAC address. Can only get internet with a specific MAC address.

For almost 30 years I've been able to do this with comcast/xfinity internet service.  I have an Arris SB6183 modem and a Netgear WNR2000v2 router (neither are comcast supplied).

Two weeks ago I changed the MAC address in my router to get a new IP like I had been doing for 30 years.  When the Modem and router restarted, I had no internet at all.   

I could get to my router config at 192.168.1.1, but I could not  get to the cable modem status page on 192.168.100.1.  I changed it back to the previous MAC and had internet again with the same previous  WAN IP address.  I could also get to the modem status page.

Next I removed the router from the equation.   I connected my first computer, that has the MAC address that works, directly to the cable modem via ethernet cable.  Cycled modem power and It worked fine.   

Next I connected my second computer with a different MAC via ethernet cable directly to the modem.  Cycled modem power and had no internet at all.   Again I also could not get the the modem status page on 192.168.100.1.   All lights on the modem appear as connected.  

After I put the working MAC back in and got online,  I logged into xfinity and went though troubleshooting although there was no option for this situation.  I choose the no internet option and it did a check of signals and modem and reported everything was normal and working.

I also tried powering down my modem and left it off overnight for 10 hours.  It still had the same WAN IP when I powered it up again.

Like I said, this all just started 2 weeks ago.