I’ve just found the solution on Unreal forums which has worked for me. Apparently as of ios14.5, Apple changed their signing requirements which meant the iOS windows signing application did not work. UE4.27 has introduced a fix and if you take the files from the engine folder …/binaries/dotnet/iOS from 4.27 and replace them in an earlier version it then should sign properly using windows remote build (remember to back up old files before replacing them). Therefore if you’re on an older version of UE4 for you app, you’re not necc forced to move engine versions.
Hope this helps.
PS - I have a feeling the 64bit thing was a red herring