Sync app on Tablet: Unable to get past security message & non-working login button

My Nextcloud server works with no problems. It uses a self-signed certificate. My system operates locally, behind Carrier Grade NAT and to the world is accessible only via a VPN. It will synchronise with my KDE desktop and my Android phone. I can access Nextcloud on all devices via Firefox with no problems. However, on my tablet, the login process, on the app, doesn’t work. I am confronted with a security message and, below that, a login button *, which will not operate. Yes, I can delve into the templates on the server to eliminate the security message but that doesn’t make the login button work. My tablet and mobile both use Nextcloud version 3.29.0.

*Below the login button is another link that does not work ‘Alternative log in using app password’.

Any ideas on how I can proceed from here will be gratefully received.

I did not find a solution to this problem, well not in the way I might have expected.
I installed ‘Nextcloud-dev’ from F-Droid and that worked. Whilst there was a warning not to use it in production, but, as the ‘stable’ version didn’t work for me, then this is the best option. It would seem that the developers have possibly fixed a bug.