Not sure what Version 2 is (do they mean the GL-AXT1800 Slate AX, Chips are the same) . Firmware is 3.x or the newer 4.x.
Are you using a wired or wireless connection?
Perhaps try a different server, I know for me it took a couple of tries to find a server that met my needs.
Sense you metioned you used Surfshark app on the computer check you DNS and network adapter settings you could be doing
Also if you have other devices connected, if the VPN tunnel is pulling 400Mbps that is divided amongst the ethernet ports and wireless adapters.
You can try messing with the MTU size. This has the potential to make it even slower or unstable.