I'm glad to hear you have the experience and technical background to tackle this - sadly, it's the exception in many cases here.
Thanks for eliminating the status of your Google accounts as the root cause. Of course, that also eliminates what would have been a clear problem to fix.
I hate to be Captain Obvious here, but there is something wrong with your home network setup. I'm not a Xfinity subscriber, but several friends are. They have older gateways, which work fine with OBiTALK equipment and Google Voice.
You mentioned a VPN. Don't try to run the OBi through any sort of VPN. Also, there should be no reason to open any ports. Of course, I don't know the unique particulars of your new router, but in general, residential routers work fine with their default settings. The list of ports you referenced is probably outdated anyhow, as it dates back to the Google "Chat" XMPP era, which is long gone. Google Voice now uses SIP and TLS.
I suppose it's possible that Xfinity has blocked some ports used by SIP VoIP, but again, I have no idea. They typically don't do that.
These are the actual ports used:
https://support.google.com/a/answer/9206518I'd personally just start all over: restore factory defaults on your gateway, temporarily remove everything from your LAN and plug into the Gateway just the OBI's Ethernet cable. The OBi itself should also be restored to factory defaults if you haven't already done so, and don't make any "expert mode" changes, aside from setting its DNS servers to 8.8.8.8 and 8.8.4.4 (Google's high-performance DNS).
If that works, then you can work from there, adding equipment.