[Fedora Linux] NextDNS not installed, port 53 busy

Hi. Recently I got problems that my internet on pc did not work anymore. So I found out DNS was the issue, I disabled NextDNS CLI client and stopped & started the networking services. The internet came back again but I am now unable to use the recommended method for Linux because when I write in the terminal:

nextdns install

it says the installation is successful and init service was also installed, but then with

nextdns status

it says "not installed"

If I ignore and try to activate it, the activation at a certain point says:

Startup failed: proxy: tcp: listen tcp [::1]:53: bind: address already in use
Error: proxy: tcp: listen tcp [::1]:53: bind: address already in use

I then did

ss -tunap

and seems like the service SyncThing is using port 53 but I am no expert in networking and no expert with linux. So I would like to understand how let everything work. I even uninstalled NextDNS with dnf remove nextdns and installed it back, but the issue is there.

I used NextDNS on my Fedora laptop since months with NO issues, but yesterday this came up. So after uninstalling NextDNS and restarting the network services I also had a moment when somehow Telegram Desktop App keep working, while Firefox won't load anything. After a PC reboot it also happened that initially all was working, but at a certain point of time (after 10min) Firefox was not working anymore (not loading any page) but Brave and other browsers would load just fine. A restart of Firefox solved the issue.

What is going on?