Synology VPN Server not externally accessible

[#1]

Hello everyone,

I'm trying to set up the VPN Server on my Synology, and while I am able to connect to the VPN locally (using my phone) when on my local Wifi, I cannot connect when trying to connect externally (using my phones 4G connection instead).

It's a L2TP VPN Server, and I've got UDP Ports 4500, 1701 and 500 all forwarded on my router (A Plus Net Hub One). When I try an online port checker, they claim those three ports are infact closed, so I assume the problem lies there, but I'm not really sure what else I can do after forwarding my ports locally?

I've done a bit of searching on here for this issue, and my external IP address begins with 83, so that seems ok. 

Has anyone got any ideas of things I could try to resolve this?

Thank you

[#2]

@srunciman check that your PlusNet Firewall is OFF https://portal.plus.net/my.html?action=firewall ( you will need to login to the member centre with your username/password ). If you have to change it then you will need to disconnect/reconnect the PPPoE connection to action the change.

edit: with the new member centre you may need to login first and then use the link

[#3]

Just checked that option, and it was already set to off. Should I still restart my router anyway?

[#4]

If it was already OFF then I wouldn't think restarting it would have any effect.

[#5]

Thank you anyway. Is there anything else worth trying? I'm totally stuck about what to do here