Double NAT problem - trying to set up a VPN to access files remotely from a mobile

Hello guys,

I have only some basic knowledge about networking and since several days I'm trying to solve this issue. I cannot replace ISP's shitty router, so I have double NAT. Everything is fine, except I cannot set up a VPN for accessing files remotely from the mobile. The network looks like this:

https://i.imgur.com/rsxvXow.png

ISP router has no bridge mode available, so on ISP router I set a static IP for USG WAN port - 192.168.0.2 and added it to DMZ. I used No-IP DDNS, since the public IP is changing dynamically and followed the tutorial of enabling RADIUS server on USG, creating user and network. Then I discovered that I cannot forward 1701 port used by L2TP, because it's reserved for another service on ISP router. Any ideas for workarounds? Maybe I'm missing something?