WiFi is just a connection between a mobile device and broadband router, that has nothing to do with any mobile network - but your SIM is required in order to connect to EE's WiFi-calling service.
Over how wide a geographical area were you not getting any service? And when did your phone last have mobile coverage outside your home?
Changing your SIM is not necessarily a fix for a loss of coverage, and has certainly made things worse now! Do you have any friends or relatives locally, who you could compare service with? And also use for a 2way SIM-swap?
If all else fails, you will either need to borrow a phone to contact EE-CS, use the live chat facility within the myEE app, or ideally I suspect... visit an EE retail store with valid photographic-ID in order to get a replacement physical SIM face-to-face. You can then troubleshoot any potential coverage issues, separately.
