Hi,
How do I access the vpn on my hAP AC lite which I am using behind my ISP’s router (Sky, UK)? Do I need to forward a port on my ISP’s router?
Many thanks
Depending on which vpn protocol you use, you do need to forward that port to your Mikrotik. Yes.
Or connect from the Tik outwards to another router where you can connect to.
If that is not possible (Firewall,double nat, etc..) , you can always use a reverse VPN.
Insteed of your hAP ac lite being a VPN-Server,
Your device is a VPN-Client of a another device.
Which was the second part of my answer 
Oh man … Sorry
Should have read it more carefully