Hello everyone!
After a very long tryhard I solved my issue!
It seems that for some Internet Service Proveder (Wind3 in my case) the default DNS they use generate this conflict with Grounded Multiplayer.
If you are issuing my same problem, try with this:
From your console, go in network property > advanced option. Then go in DNS setting and setup DNS manually, using for example the google dns 8.8.8.8 / 8.8.4.4.
If your domestic network is composed by more than 1 router and you use the DNS settled in this way, you may sometime issue the "limited NAT" warning, but in my case I can still play the following multiplayers game very well: Deathloop, Sea of Thieves, fall guys, Halo infinite, Fifa 22.
Don't know if this is the generic solution but I hope it can Help someone.
Greetings,
Zell