Hi Micky,
I have a B200 first registered in January 2020 and my app works perfectly plus I have just changed my phone to 5G and it connected straight away with no issues As suggested before try uninstalling the app and then re installing but only if you have the original password and I believe pin.
I would also suggest trying a different dealership as they should be able to sort it easily enough for you if you have one within a reasonable distance.
Also ring UK customer service on 0207 660 9993
Hope this helps.