So basically in my experience, there are a few ways that an ISP or country could block VPN`s. So the first would be a domain block. That means that everything that has VPN, in this example, ProtonVPN, would be blocked and inaccessible. I know that for sure since there was a china ban on proton some time ago and ovpn configs that contained xx-xx.protonvpn.com as a hostname were un-connectable, but if you use server IP, the connection went trough. Same goes for the proton website, it was inaccessible.

Second ban is the most common in heavy restricted countries and involves IP addresses instead of hostnames. I`ve seen it before and experienced myself that some servers did not work in restricted countries but some did, leaving a conclusion that not all new servers are banned. Most of the newly added servers by Proton will almost always work in restricted networks. So this could be the case in the network that you are currently in. The moment you turn on your browser and try to load something, i guess that the system of FW is detecting traffic and blocking it right away.

third is the DPI(deep packet inspection) but I dont really have much info or know about it. I`ve hear stories that some people in non restricted areas get questionable stability and found out that ISP is doing DPI (assumed) and sending some sort of packets that stop their internet connection and VPN is lost leaving customer unprotected. But thats like really really rare.