Odd remote viewing issue

I’ve run into an usual situation and I’m hoping someone here has experienced and resolved this. It involves Reolink cameras at 2 locations. I have 12 Reolink cameras of all types and an NVR. I have never had an issue viewing anything remotely, no matter where I am. I have Comcast/Xfinity internet with my own modem and firewall. So my stuff is all good.
My son just bought a house a few miles away. While we are waiting on furniture delivery, we got internet setup and installed some basic smart home infrastructure (which all works fine) along with 3 Reolink Lumus cameras (PoE or wired is not an option as there are no Ethernet cables and only a few coax runs). He has Comcast Internet (500/100) with the Xfinity Gateway XB7. The Lumus cameras work perfectly in his house and have strong WiFi connections.
Here’s the issue: in his house, on his WiFi, the Lumus cameras work fine, but we can’t access my cameras (“connection failed”). When we are at my house, it’s the opposite - mine work, can’t see his. On cellular away from both homes, mine work fine, his work sometimes but eventually fail. He put smart plugs on them and can power cycle them remotely when they are unreachable, they come back but become unreachable again after a short time. Once when they were all unreachable, we drove down there and as soon as we got within range of his WiFi, they were viewable. I am confident the cameras themselves are not dropping off the network, we’ve been there for hours getting other things setup and they are rock solid. All his other smart-home stuff works fine and is reachable from anywhere. It’s just viewing the Reolink cameras remotely that are the problem. My theory is it’s something in his Xfinity Gateway possibly blocking random UDP ports, or maybe related to UPNP? And yes, UID is enabled on all cameras. Anyone experience anything like this? Sorry about the long winded post, but I wanted to get all the information out there. Thanks!