@Robull 

Hi,

To find out if the problem lies within the AX90 you can do the following test.

Connect your device that is running the PPTP VPN client to the ISP's router (either via Ethernet or, if the ISP's router has Wi-Fi, via wireless) and then enter the WAN IP address of the AX90 in the PPTP VPN client and then click "connect".

If a connection is established successfully, then there is no problem with the AX90 and you have to look at the ISP's router again.

One important thing to verify on your ISP's router would be that its Internet IP address is indeed a public IPv4 address and not a private one.

@woozle 

I have checked your suggestion. I can connect to the AX90 router from the ISP router through it's local address, and I can confirm that the ISP router has a public ip4 address, and also a public ip4 address. Do you have any suggestion, what should I do on the ISP router. I've tried port forwarding, but that didn't seem to help, both PPTP and L2TP. The only thing I don't know how to do is setup the required IP protokoll.

@Robull 

I haven't used PPTP VPN for years, but I think forwarding of Port 1723 / Protocol TCP should should be enough, although some say that Port 47 / Protocol ALL (or actually "GRE", if it's available) should also be forwarded.

Also check on the ISP router if it has a dedicated PPTP Passthrough option. (as an example, the picture below shows what that looks like on a TP-Link router)