There are some free ones, the best one (but still not great if you don't pay) is ProtonVPN imo, or you could just use the refund policy after subscribing to nordvpn or other big names if it's an emergency.
The reason it works is because WC (and a lot of other companies) blocks IPs that aren't considered secure enough and I believe it has to do with a 4g hotspot's DNS settings or something like that. (I read about it a long time ago, don't really remember and it is not my field of expertise so I might be off on a few things). Using a VPN means your "low security" connection goes through a server that itself doesn't use 4g and its access ports but rather "regular" means of sending information, which WC considers valid. Either way, it works everytime.
Last note I wanna say this makes the game quite laggy so don't expect to be raiding on a 4g + VPN combo but if your internet dies on you and you need to feed some baby gigas asap it's a good option 😛