Kobo don't use the email for registration unless you buy on their website. Also it doesn't even need to be a real email address. I created a real unique to Kobo email address about 6 years ago for registration and only ever received my own test email on it, so they don't give the email address to anyone.
Reset and put in either a real or fake email address. Then you can make SideloadedMode=true and never use WiFi.
Use either 4.32.x or 4.34.x fw
Older FW via button at bottom
https://pgaskin.net/KoboStuff/kobofirmware.html
I've set up a Nia I bought for someone.