If you are using ProtonVPN for your cell phone, you can also use it on an Ubuntu Linux installation. Here is the official guide, and I can confirm that it works and the guide is excellent.
Essentially installation on Ubuntu 20.04 involves going into your Ubuntu settings and selecting “network”, and hitting the “+” at VPN. From there you will “Import from file” and select the VPN config file you downloaded from the official guide. Once this file is selected, you can enter in your ‘OpenVPN / IKEv2 username’ from the ProtoVPN Dashboard “Account” section. Your new VPN configuration is ready and can be selected in the upper right-hand network icon.
distantdark 
All this worked very nicely for me, and now I can see another ip interface by issuing the “ip address” command: tun0. From this I conclude: I am connected.
But if I open now my Firefox and go to whatismyipaddress.com – it still tells me my “normal” ip address. From this I conclude: Even though I have now that VPN tunnel set up properly, Firefox is not using it.
So my question is very simple: How can I ensure that the VPN tunnel is really being used from my Ubuntu Linux system?
Sorry, I may have asked the same question already, but I do not find it any more!?
So again: I did all the steps successfully, I am connected to the ProtonVPN on my Ubuntu 20.04 LTS, which I conclude from the fact that the “ip address” command gives me a “tun0” entry.
But then, if I am using Firefox for surfing the internet, I seem to be at the exactly same internet location than before – according to websites like whatismyipaddress.com. I do not understand this, because surfing “from another location” would be the most important reason for using a VPN at all – no?
In the past I used VPN, another system, from my Android phone, and once I was connected to VPN, all that I did went through it.
Now, with Ubuntu, I did not find any way to somehow “configure” here or there and get it working, so I am confused…
Is there something basic that I am still missing?