The phones need to pull firmware for SIP, you basically just told the phone to go into SIP mode, now you need to give it the appropriate firmware, language file, and configuration files via TFTP. Unfortunately I don't think the 5320 has (or ever had) a SIP mode, but it uses a unified bootloader similar to the 5330/5340 sets which do (or did) have SIP functionality, thus that options still shows in the menus but it should NEVER be used. You likely will never get a 5320 to work for this application.
TBH, I would never use a Mitel MiNET set for SIP by choice, they haven't been supported for MANY years for this application... get a Polycom, Mitel/Aastra (Aastra models only!), Yealink, Grandstream, snom, Panasonic, or some manufacturer who has support for generic SIP applications, plug in your IPs and credentials and enjoy.