Hi - welcome to the forum.
Has it ever worked on the soho? Did it work on the router you had before perhaps? Where is the Avaya voip service itself? Is it also on the LAN or are you connecting to a cloud or HQ hosted service?
You could try setting the SIP ALG to compatibility mode on the SOHO. Apart from that, it will very hard to fault find. Check the phone has an IP address from the Soho and you can ping it, and if it ha an IP you’ll need to go to the logs on the PBX to see what’s up.