Improve Wifi Reception for wifi calling


I mainly use wifi calling, and I send SMS over wifi too. It appears that, without changing anything, at the same place, it works more or less, depending on the load on the wifi server I think ?
But most often wifi calling does not work whereas the wifi is strong enough for WhatsApp or Signal calling
I should precise that I use a common wifi box, so I have no access to it
Following some tips, I could improve the wifi calling service using a VPN (Proton VPN) with a P2P server, and UDP protocol (I should precize that I mainly don’t understand theses terms, I just followed some tips)
But it then depends on the load on the VPN server
Do you have any for improving the wifi calling service without access to the wifi box ?
Thank you in advance for your help,
Thomas M