Somehow none of the devices on my home internet are able to acces apps
Do you mean none of your devices can access any internet apps on the home network or Just the Nextcloud app you just tried to install?
If it’s the former, those instructions would not have created such a result. Um restart your router then contact your ISP?
If it’s the later the instructions should work except use stable18 instead of stable13. Also php7.2 is now standard on Ubuntu 18.04 you don’t need to add the ppa. Also you either you need to own the domain OR if you’re just setting up for local testing you need to edit /etc/hosts file. For example.com to work on just the local device you would add a line: 127.0.0.1 example.com.
To access on your home network a few more steps are required.
If you’re trying to learn Linux some basics wouldn’t hurt including LAMP stack and basic networking stuff - I think linuxacademy.com has some free courses. Also check out LXC if on Ubuntu - great for setting up dev servers just to play and learn with.
Finally for easier installs, check out the Linux snap package (I use myself) or for a guided server install https://github.com/nextcloud/vm (haven’t tried myself).