I’m currently living in France and I’m settling in UK next week. So before arriving, I tried to open a Monzo account yesterday but unfortunately my request was declined without any more details given.
If you put that on the form when signing up - more than likely that’s the issue.
You may want to try with another bank, say Starling and not mention that. You’ll be a UK tax resident when you eventually use the account anyway. Starling and Monzo are very similar and have pretty much the same feature set.