I think you Just need nginx with one module: https://nginx.org/en/docs/http/ngx_http_dav_module.html That is filestorage only. WebDAV client exists for all platforms.
UPD: if you need web client you can use something like https://github.com/mgoltzsche/file-service/
In general, this is of course 100% based on speculation, but I assume that you can connect to a VPN but you cannot access any resource inside the local network. Just because this is the most basic mistake that beginners usually make. And it's related to the fact that your pi is not your default gateway. To solve this, you need to enable SNAT on the local Pi interface. There are many guides on the Internet on how to do this.
P.S. Unfortunately, I have a poor telepathy skill, so it would be good to get a little more background. At least an accurate description of the problem. =)
Yeah, that is a big deal. We have 4 days for celebrate. From 1 to 4 may. Municipalities organize fairs, concerts, etc. But unofficially, these are barbecue days. everyone goes out into nature and roasting meat on grills. And after that we celebrate Victory day from 8 may to 11 may. So many people get 3 days vacation between holidays and don't work 2 weeks. So, that is like China new year or Japanese golden week.