Thought I'd tried that approach before for Syno Certs.
Will again and leave the router to own 443 - keep my grubby mitts off!
If Syno Cert process can tell Let's Encrypt to respond on 4443 or whatever, then another say Syno could use 5443, etc. and they not squabble over 443.
Every day is a school day!
Ta. Onward...