I do have main router (TP-Link Archer) that is connected to internet.
I have a spare D-Link with OpenWRT that I'd like to use as temporary access point and VPN client - i.e. I'd like to have a separate wi-fi network (although can be cable) where all traffic is going to some VPN provider (I have *.ovpn file from them).
I'm new to OpenWRT and is lost: how can I do that?
(I tried searching site and forum)
You will just setup your OpenWrt device as a normal router and connect it to the upstream. If you can connect a wire between the upstream lan and the wan of your device, that's the easiest. Otherwise you can use it as a wifi client.
You may need to change the lan address of your OpenWrt router if the upstream one uses the same subnet (192.168.1.0/24) since that will cause a conflict. If it's not, you should be good to go.
From there, setup wifi and verify that your client devices can reach the internet.
Finally, setup your VPN. Sounds like you're going to be using OpenVPN based on the file you have in hand, although if you can use WireGuard, that will likely be much more performant.