I can ping PPTP VPN-server from router, but the clients have no connection

[Mastiff]

I have a Windows server at home with PPTP (yes, outdated, but please, only answers to the question, not "you need to go to Wireshark or OpenVPN or whaterver"), and I have set up VPN on an RT-AC88U, which is my goto router. I have bought a bunch of them. I can now ping the server from the router's Network tools interface, but not from any clients connecting to the router. And actually nothing works between the client of the router and the Internet when the VPN on the router is activated. It comes back at once when I deactivate it. I have tried to deactivate WAN -NAT pass-through for VPN, but I was pretty sure that wouldn't change anything, and it didn't.

I don't think this is a network segment problem, the router has the LAN address 192.168.3.1, and the WAN address 192.168.5.1. That is coming from a wireless client connecting to the AirBnB-host's wifi, with the LAN address 192.168.5.1. The one possible caveat is that the Airbnb's host network is on 192.168.1.1, and that is the WAN segment of the VPN server at home. But I don't think that should matter because I can use the Windows VPN client on the laptop without problems. I just want it to work on a few other devices.

Can somebody please tell me what I need to do to get this working?