AC1900 loses internet access after reboot completes, need to manually disconnect/reconnect to internet each time

[LabQa]

Doesn't appear anyone has asked but what is the model number of the Cisco modem? Is it an actual modem or is it a wifi router/modem/gateway? If you have a Cisco router/modem combo or gateway you will typically need to put it into bridge mode if you plan on using your own router behind it.

https://www.xfinity.com/support/articles/wireless-gateway-enable-disable-bridge-mode

The firmware you appear to be using is 3.0.0.4.376_3221 which is likely the stock (very, very old) firmware that came with that T-Mobile Asus router. Don't remember off hand if one can update the T-Mobile Asus router to later firmware without having to modify the router's code which generally cannot be discussed on this site.

Been years since dealing with Comcast (Xfinity) but if I remember right it used to be you had to "clone" the MAC address of the computer that was initially used by the tech installer/self install when setting up the service. It used to be the cable company linked the MAC address to the account. One may have to call Xfinity support and see if they can release the MAC address, if tied to the account, so you can use a different one if the clone MAC address option in the Asus firmware doesn't work.

The modem is a Cisco DPC3008 (no WiFi), and the router firmware is the latest available for the TMO branded AC1900.

I just removed the Mac address under "Special Requirement from ISP" section in WAN and rebooted the modem and lost connection but this time had a different error "DHCP Connection Failed" and wasn't able to manually reconnect (even after router reboot). I then put back the Mac address (now using the current computer Mac address) and after modem/router reboots and manual disconnect/connect regained web access.
So, without a Mac clone address I can't get any access at all, and now with an updated Mac address I continue to have the issue of losing connection after router reboot and having to manually reconnect.

Maybe because it is an old setup (around 2015), as you noted I need to talk to Xfinity, perhaps something is tied to my account (and maybe because I am not using their modem/router?) that still requires my router to provide a Mac address....and maybe that is somehow causing the disconnect issue after router reboot.

 

[drinkingbird]

The modem is a Cisco DPC3008 (no WiFi), and the router firmware is the latest available for the TMO branded AC1900.

I just removed the Mac address under "Special Requirement from ISP" section in WAN and rebooted the modem and lost connection but this time had a different error "DHCP Connection Failed" and wasn't able to manually reconnect (even after router reboot). I then put back the Mac address (now using the current computer Mac address) and after modem/router reboots and manual disconnect/connect regained web access.
So, without a Mac clone address I can't get any access at all, and now with an updated Mac address I continue to have the issue of losing connection after router reboot and having to manually reconnect.

Maybe because it is an old setup (around 2015), as you noted I need to talk to Xfinity, perhaps something is tied to my account (and maybe because I am not using their modem/router?) that still requires my router to provide a Mac address....and maybe that is somehow causing the disconnect issue after router reboot.

They haven't required a specific MAC for about 20 years. Rebooting the modem should be all that is required to swap the connected device.

Maybe that router somehow got blacklisted at some point for abuse or something. Or it is a bug in the firmware.

If you unplug the AC1900, reboot the modem, then plug a laptop or computer in, does it immediately get an IP and have internet access fine? Try plugging and unplugging the ethernet a couple times without rebooting the modem too. Just trying to rule out potential issue with the modem.

It is entirely possible that there is just an incompatibility with that really old firmware and the newer Cisco modem. I doubt it is any setting on Comcast's side. You can try to see if you can find newer firmware but as far as I know that router updates itself and you should see a flashing light when in the GUI telling you a new one is available if that is the case. But go on the firmware page and click the "check" button to see. If you really don't want to get a new modem, ask them for a different brand modem. Heck if they're charging you for a modem they should be leasing you a modern gateway and you can ditch the T-Mo all together. Not sure if you own the modem or not.