OnePlus Nord 2T and Smarty

I have a OnePlus Nord 2T phone, which I got as a brand new phone a while ago but only just trying to use it with my Smarty sim card for wifi calling. I have read some other posts that seem to concurr that it doesn't work! I am wondering if anyone has found a way to get it to work? I have been using a OnePlus 6T which also wasn't supposed to work, but I found some software online that I risked downloading and it has been working fine for a few years. I only wanted to upgrade for a better camera, I like my old phone!

I contacted OnePlus and they just said Smarty wasn't a compatible sim. Seems very odd when I've been using a much older phone without issue (granted with additional software). 

I live in a remote area with no mobile signal so wifi calling is absolutely essential for me. Grateful for any help!