Maybe you do not have IPv4? Depending of your IP, your phone maybe IPv6 only despite the Dual IPv4/6 setting on the phone. Normal browsing, e-mails etc. works. Other symptoms of missing IPv4 are a non working WiFi hotspot. If the VPN file contains server’s IPv4 address, that would not resolve. I resolved the FQN name of the server’s IPv4 address on my computer and put the FQN (name, not IP-address) in the VPN server description file (not Proton, in my case). With that the VPN is working again, see my somewhat lengthy explanation in the above thread.