Panasonic VOIP phones only work for ~30 seconds before saying 'Poor LAN connection'

I'm hoping someone here can help me because I feel like I'm banging my head against the wall trying to solve this issue.

So the setup: Main building with a Sophos UTM 9, firmware version 9.707-5 with the Panasonic PBX behind it. Secondary building across the street, also with a Sophos UTM 9, with firmware version 9.708-6 and 4 Panasonic KX-NT343 VoIP phones behind it. The two firewalls are connected via IP sec vpn tunnel. While at the secondary building, I can ping the PBX just fine, the Panasonic phones get an IP address and I can ping them. I can make outgoing calls from the phones, but only for about 30 seconds until the phones reboot themselves with an error message saying that there is poor lan connection. I was able to verify with Sophos support that there is traffic flowing from the phones to the PBX and vice versa whenever I make a call. There's nothing wrong with the phone system either, as I took one of the phones from the secondary building to the main building and plugged it in and it worked right away with no issue.

If it was just one phone, I would say it's a bad phone and replace it. But it's all 4 phones at the secondary building, all having the same issue. I've called Sophos support 4 times now, spending multiple hours on the phone with them, with no change. This started happening out of the blue several days ago and I haven't been able to figure out why as I had made no changes to either firewall for several days before this happened.

The only thing I can think of now is that the firmware versions aren't the same? But like I said, worked for who knows how long before this with two different firmware versions. Any advice would be appreciated, before I pull out what little hair I have left.