The domain makes proper security certificates for HTTPS much more usable and easy to manage.
The reason you are finding it hard to use IP only is because yiu have to disable certain security features ti make it work.
You can get a domain for $1 per year. I would highly suggest getting a domain and setting up a proper reverse proxy which will allow to to easily setup any service you want after that.
Nextcloud is also one of the harder things to setup.
You could start with something easier like a simple dokuwiki site as a test to get the reverse proxy and domain name part working.
There is a reason why this is the "correct" way to do this. It will make all future projects much easier.