Cannot load specific websites when using the gateway

Try disabling the Xfi Advanced Security feature as a work-around for now and see. It's buggy ! And there is currently no way for an end-user to be able to whitelist anything;

https://www.xfinity.com/support/articles/using-xfinity-xfi-advanced-security

YMMV. Good luck !

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@babao wrote: ... It's working fine with cellular or when I remote into the office computer.

traceroute shows problems ...

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I can reach the site. When one customer cannot reach a site but others can, and you cannot reach the site using your Comcast connection but you can using others, it can mean the site is blocking your public Comcast IP. You can look up your IP at https://www.google.com/search?q=what+is+my+IP. You'll probably need to ask the hosting service admins for help with this.

 My trace looks like this:

C>tracert -4 babaosoftware.com
Tracing route to babaosoftware.com [107.180.20.71] over a maximum of 30 hops:
  1    12 ms     3 ms     3 ms  router1 [192.168.1.1]
  2    13 ms    13 ms    12 ms  96.120.9.181
  3    14 ms    12 ms    13 ms  96.108.119.45
  4    13 ms    13 ms    14 ms  96.110.24.214
  5    17 ms    11 ms    11 ms  96.110.25.65
  6    23 ms    22 ms    25 ms  be-34-ar01.mckeesport.pa.pitt.comcast.net [69.139.168.141]
  7    35 ms    34 ms    25 ms  ae-7.bar1.Pittsburgh3.Level3.net [4.68.106.85]
  8     *        *        *     Request timed out.
  9    31 ms    30 ms    30 ms  ae6.ibrsa0102-02.iad1.bb.godaddy.com [4.15.136.122]
 10    30 ms    31 ms    30 ms  ip-148-72-36-47.ip.secureserver.net [148.72.36.47]
 11     *        *        *     Request timed out.
 12     *        *        *     Request timed out.
 13     *        *        *     Request timed out.
 14     *        *        *     Request timed out.
 15     *        *        *     Request timed out.
 16    28 ms    28 ms    31 ms  ip-107-180-20-71.ip.secureserver.net [107.180.20.71]
Trace complete.

Nothing worked so far. It's working fine with cellular or when I remote into the office computer.

traceroute shows problems, as it cannot find the route. Even when I use the xfinity modem troubleshoot screen, it finds the domain, it finds the ip address, but it fails on traceroute. I feel the problem is there, but I don't know what it can be.

Here is the traceroute output. It will keep on timing out after 12 hops

% traceroute babaosoftware.com

traceroute to babaosoftware.com (107.180.20.71), 64 hops max, 52 byte packets

1  192.168.86.1 (192.168.86.1)  8.535 ms  3.076 ms  2.095 ms

2  10.0.0.1 (10.0.0.1)  3.303 ms  3.720 ms  3.289 ms

3  96.120.64.209 (96.120.64.209)  8.905 ms  9.548 ms  10.303 ms

4  96.110.29.101 (96.110.29.101)  11.325 ms  10.539 ms  10.220 ms

5  162.151.191.34 (162.151.191.34)  10.572 ms  11.461 ms  9.450 ms

6  162.151.191.21 (162.151.191.21)  12.470 ms  11.888 ms  12.665 ms

7  be-2-ar01.needham.ma.boston.comcast.net (68.87.147.149)  15.322 ms  16.213 ms  15.693 ms

8  * * *

9  * * *

10  ae6.ibrsa0102-02.iad1.bb.godaddy.com (4.15.136.122)  27.175 ms  30.775 ms  30.613 ms

11  ip-148-72-36-47.ip.secureserver.net (148.72.36.47)  29.149 ms  28.474 ms  28.381 ms

12  * * *

When I do the same on the remote computer, it finds it, no problem (it's a windows machine, so I use tracert):

tracert babaosoftware.com

Tracing route to babaosoftware.com [107.180.20.71]
over a maximum of 30 hops:

1 1 ms 1 ms 1 ms 172.30.244.3
2 2 ms 2 ms 2 ms 3650-int-b.ibi.com [38.96.131.1]
3 5 ms 3 ms 3 ms 42.tengige0-0-0-0.gw5.nyc4.alter.net [152.179.16
1.233]
4 1 ms 1 ms 1 ms 0.ae1.br2.nyc4.alter.net [140.222.229.91]
5 1 ms 1 ms 1 ms verizon.com.customer.alter.net [152.179.78.154]

6 * * * Request timed out.
7 7 ms 7 ms 7 ms ae6.ibrsa0102-02.iad1.bb.godaddy.com [4.15.136.1
22]
8 8 ms 8 ms 8 ms ip-148-72-36-47.ip.secureserver.net [148.72.36.4
7]
9 * * * Request timed out.
10 * * * Request timed out.
11 * * * Request timed out.
12 * * * Request timed out.
13 * * * Request timed out.
14 7 ms 7 ms 7 ms ip-107-180-20-71.ip.secureserver.net [107.180.20
.71]

Trace complete.

I did asked the hosting, which is godaddy.com (by the way, the site is mine). Their tech reps are as puzzled as I am. Also, spent hours with xfinity, nobody can figure out why. GoDaddy checked the access from 6 various locations, all worked. Their best guess was that somehow my ip address assigned by xfinity to my modem, might be blacklisted somehow. I am totally out of ideas.

Actually there is one thing I can do, exchange the modem. xfinity told me that that will change the ip address.

Just for further reference. I have not replaced the modem yet (in order to get a new ip address), but the ip address is definitely the cause of the problem. I know this because if I use a VPN, like Express VPN, everything works just fine. I'm guessing my hosting company, GoDaddy, has somehow blacklisted my current ip address, even though they dont' admit it, and the only way to change it is to get a new modem.

FWIW, placing the Comcast supplied gateway device in to bridge mode and using a separate router with it will get you a new / different IP address.