I'm posting this with an answer because I could find no solution on either xfinity or linksys/velop forums. This configuration is between a Linksys Velop model WHW03 and an xfinity modem/router model TG1682G. For many months starting July 2018 - I successfully ran a "mesh" wifi network via the Velop system by setting the xfinity modem/router into "bridge" mode and connecting the master Velop node (1 of 3 nodes) via an ethernet cable to it. Everything worked perfectly until March 2019, when after a power outage, the Velop master node could no longer establish an IP address via the xfinity modem/router and therefore, could not connect to the internet. I also noted during troubleshooting that even a laptop "hard-wired" via an ethernet cable to the xfinity modem/router was not being assigned an IP address while in bridge mode. For this reason, the only way I could get my xfinity modem/router to disable "bridge" mode was to call support and have them do it. Long story short, the Velop system does not require bridge mode. I called Linksys and they had me restart the xfinity modem/router, then the Velop master node hardwired to it, WITHOUT CHANGING the xfinity router into "bridge" mode. The Velop master node was then assigned an IP and was able to connect to the internet. So... after confirming all was well with the other two Velop nodes, I connected to the xfinity modem/router and disabled it's WiFi on both frequencies while NOT enabling "bridge" mode and all is working from my Velop WiFi now. Hope this helps someone.
I experienced the same problem when I had a power interruption about a month after I installed my two Velop routers (12/20/2018). After contacting both Comcast and Linksys tech support, after many hours, Linksys tech support arrived at this solution:
1) Turn off power to both the Comcast modem and the primary Velop router.
2) Disconnect the ethernet cable from the Velop router or the modem
3) After about 10 seconds, turn on power to the Comcast modem
4) When the Online indicator light comes ON and is steady, reconnect the cable to the Velop router and turn the Velop router ON. (I wait about 30 seconds after the Comcast Online light is ON before turn on the Velop router.
I have had two power interruptions since the first time. Each time the Velop router lost the ip address. Each time I followed steps 1 thru 4 above to get an ip address from the Comcast modem.
I used an Apple Airport router connected to a Comcast modem for over 10 years and never experienced a problem after a power interruption. I think that the Velop router is requesting an ip address from the Comcast modem too soon, i.e. before the Comcast modem is fully online after a power interruption. When it does not get an ip address, it timesout.
If I am not at home when a power interruption occurs, my wife would be without internet service.I hope that Linksys will fix this issue with a firmware update.