Yeah, great! DNS is ok
Is the IPv6 that you see in the diagnosis the same IPv6 as in the screenshots, ending in 57bd?
I used to have a Fritz!Box (for ADSL, later fibre, not for cable but I expect them to work the same - it looks the same anyway) and it worked. Sometimes with some trouble, but mostly it was OK.
I’d say the forwardings in the screenshots look good.
When you use the pencil to edit a line, you can disable the forwarding temporarily (instead of deleting it with the red cross). If you do that for the IPv4 forward on port 443, can you still reach the Yunohost from outside the lan? That would imply IPv6 is working, even though Yunohost-diagnosis gives an error.
As an aside, did you know about fritzchecksum? When you got sick of using the Web-interface of the router, you can make a back-up, edit it, calculate the checksum, and upload it to the Fritzbox. The format is a bit peculiar, but more comfortable to work with if you want to edit mulptiple things. Keep a back-back-up, it is sensitive to errors.