IPv4 address keeps resetting to local IP behind ATT BGW320-505 in passthrough

[RegularSizeRoss]

I have a weird issue where my IPv4 address for the WAN interface will periodically switch over to an IP on the local DHCP network from my BGW320-505 ATT router. The router is setup in passthrough mode as per all the guides, the only device on the ATT router is my OPNsense box. Whenever this happens I lose external network connectivity (makes sense since I don't have a public IP, just the local address from the ATT router). I fix the issue by going into Interfaces > Overview > WAN interface and clicking the DHCP "Reload" button, which will give me the public IP again.

This only started happening recently, but I can't peg it to an upgrade or any specific system change. It will be fine for a few hours (and actually for around a week this last time) but then a website will fail to load and I'll go back in and inevitably see the local assigned IP.

The IP is definitely coming from the ATT router DHCP because I changed the network to 192.168.10.x (which I don't have on my OPNsense box) and the interface gets an IP on that network. I think it's an OPNsense thing because the router hasn't changed/updated and I was running my box successfully for about a year before this started happening.

Other things I've tried: "Reject Leases From" setting, but that makes it so I can't get the public IP either. Turned off any sort of firewall settings in the ATT router. Made sure it was the only device available on the ATT router.

Thanks!

[CJ]

What did you put in the Reject Leases field?