Can't run VPN on router as client.

[#1]

I'm trying to set up Surfshark as a client on a Asus DSL-AC750.
It connects but the Conenction Status icon never loads and the VPN isn't appllied.

I've spent over five hours with Surfshark customer support* only for us to be no closer to understanding why, (now waiting for an email from their dev team). 

Has anyone else had a similar issue or do you know anything that could shed any light on this for me? It's driving me insane.
Thank you.

* Off the top of my head we touched on DNS, Firewalls etc, NAT Passthrough, VPN protocols and IPv6 was asked about several times (its always been off).

[#2]

I have SurfShark client running fine, never tried it running on a router though.
One thought, do you have safeguard switched on?

Also assume you've got the Asus connected to a modem and not a Hub1 or something?

[#3]

I just checked and It is off, alongside the breadband firewall etc.
Thank you for replying through, all suggestions are much appreciated!

AC750 has a built in modem.

[#4]

Is it compatable with Surfshack, or have you installed WRT firmware?

https://surfshark.com/blog/setup-vpn-router 

[#5]

Thanks for replying.
Yes it is all supposedly compatible and the steps in the guide you linked are the same as I've followed.

I'm pretty sure I've seen accounts of people doing the same thing successfully with the same ISP/Router, Router/VPN and VPN/ISP independently. Also a lot of Surfsharks guides revolve around the same firmware as my routers', and Surfshark is seemingly usually very good with Plusnet.