Specify peer WAN in multiwan setup

[ddbnj]

I am trying to configure a backup site to site VPN using an LTE modem. I have this setup working in openvpn because I can specify "The interface used by the firewall to originate this OpenVPN client connection"

Can this be done in Wireguard for pfsense? The caveat is that the LTE modem functions as a cellular bridge so the IP address is dynamically assigned behind a NAT. I tried creating a gateway from the wireguard interface but since I need to specify the ip address, I figured that solution would be short lived.

I feel like I'm missing something obvious.

Thanks for the help.

Devan