You should ask Matrix themselves about this since it isn’t specific to Nextcloud. 
Try their main chat, or search their room listings for other support channels:
#matrix:matrix.org
Also, try internet searching for your federation /var/www/html/.well-known/matrix/server issue. Seems it is covered in setup documentation online. Says you must add a srv record to index.html
Try following the documentation directions and confirm you’ve gotten SSL working on your matrix subdomain, then adding the serv record to index.html and then checking whether federation works.