Thanks for your reply!
I am beginner when it comes to this setup. But to help understand my setup looks like image here-
I will be accessing using WAN IP of Vultr VPS (B). Might use DynamicDNS to this IP.
Then nginx running on Vultr VPS should forward through tinc tunnel to RPI4 nextcloud (A) at 10.0.0.2 tinc VPN address.
Also, when I am in home network, I want to access (A) directly using LAN IP preferably over https.
Hope that clarifies a bit more about my setup.
Right now I also have a problem with nginx-
If I open Firefox in VNC running on Vultr VPS (B) and type 10.0.0.2 I can see Nextcloud login.
(10.0.0.2 is in trusted domain).
But when I forwarded using nginx proxy_pass :80 to 10.0.0.2:80 and then try to open WAN_IP:80 from any browswer, it just keeps loading and finally fails. WAN_IP:80 is changed to 10.0.0.2:80 in browser.
Any help highly appreciated!