If you have already opened port for your VPN, I think the problem is not related to the NAT, but you can try with the PPTP ALG enabled and disabled. And may I know the exact topology of your network?

I've tried with and w/o ALG.
This used to work at some point. I do not know what changed. It doesn't work if I forward PPTP (1723) to the Linksys or Synology
I should add that frontier router is just a modem. it no longer does routing (the picture is wrong). It's bridged with TL-R470T+.
I couldn't bridge wave2net connection, as i don't have login

Hi

I have kind a of the same problem and from what I have read we need to enable some GRE Protocol 47. I am still trying to find out how to do that. I have the R480t+ but it should be almost the same so .... it is really strange.

It is crazy ...

As we can see there is also NAT devices in the WAN side of the TL-R470T+, so you need to open port 1723 for TL-R470T+ or set it as DMZ host.

if I do a port scan, it shows as open.
The TP-Link controls one of the connections fully (as it's bridged).
The issue is, the VPN doesn't work, if I use EITHER of the connections or EITHER of the devices behind it. If I do a VPN test on the local network it works. So the issue is in the TP-Link.