VPS to mimic ISP public ip on WAN, is it possible?

Is it possible to use a vpn (openvpn, wireguard etc) or any other sort of a "tunnel" on VPS to somewhat mimic public ip on WAN port? Guess I can call it an offshore wan..
Have a "specific" device sitting in between my modem and openwrt router, it checks if the wan has a public ip assigned, to work properly.

I guess it's possible but you would need to install the VPN gateway between the router and the "specific" device. Which means you need two OpenWrt devices. You also need one additional public IPv4 addresses on the VPS, if you want to have IPv4 access on the VPS after routing one address to the OpenWrt router (this also requires proxy arp). Or you can use NAT on the VPN gateway which is easier, and then one address is enough.

I guess, it's then the question is, mostly, how to set up the vpn to give a public ip to a client?

Or you can use NAT on the VPN gateway which is easier, and then one address is enough.

Would it not cause the router to get a private ip of the vpn gateway?

I was thinking that the VPN gateway can assign a public IP address via DHCP but use masquerade/SNAT to rewrite to the source address to an address in the VPN subnet before forwarding to the VPS.

You'll need to squat on a public subnet that's large enough to contain the public IP address, and the network and broadcast addresses. Maybe it's possible to use a /31 peer-to-peer subnet which means you only squat on one address (to be used by the VPN gateway) that's not yours.

Obviously it's not recommended to use addresses that aren't yours but I don't know if you can use a /32 subnet with dhcp,

Thanks, seems like a lot of research to be done.

Just use VPN (IKEv2, OpenVPN or whatever is your favourite) and port redirection.

guess wireguard will be the fastest? Need to push around 150mbps..

and port redirection

mind to elaborate, from where to where?