Shared Address Space WAN and /30 subnet

[Drik]

I have T-Mobile provided router with SIM card set in transparent bridge nad it gets DHCP IP from Shared Address Space with /30 subnet. When connected to other devices it is transparent and all is well, device is getting the IP and gateway. When connected to OPNsense the WAN IP and gateway are the same and they are both WAN IP. The internet is working on WAN interface, but not on LAN. When i try to temporary set WAN IP to static with /30 subnet I get an error: "This IPv4 address is the network address and cannot be used". This is fresh install and when trying to use other provider with local IP and gateway everything is working correctly. Why is it happening?