Try install a web server like Nginx. I think the Raspberry Pi OS is based on Debian, so sudo apt install nginx should work. Then hit the Pi’s address (no port number needed) and it should show a default page.

Phones sometimes will ignore your local DNS (if any) and still use whatever the vendor hardcoded.

For what it’s worth, this is mostly for security reasons - they’re using DNS-over-HTTPS so that the DNS requests are encrypted.

If you’re using the app on the phones, try the website instead?

Is it just Nextcloud having issues? Can you access other services on the Pi from the phones?

Seems like BASIC but with different keywords :D

I like this part of the readme:

How to use

Please don’t.