OpenWrt router as client to Openvpn

i would like to connect my Openwrt as client ton an Openvpn schema. He has provided me a single Openvpn key (passwordless) that i would like to connect to his network. i have tried from Windows laptop by using the Openvpn gui and i can connect fine. i would like to import that to the Openwrt and have a permanent connection for all of my LAN. So from any PC of my network to have access to his LAN and the opposite. When connected from Windows, what is my ip does not show my IP, rather hos IP. I would like to change that and keep Internet traffic from my connection. Through VPN only the traffic for the other LAN. No worries about security issues between the 2 LANs as from my case it is a dedicated router and LAN for this job.
Could someone help me how to configure that please?

Have you studied the wiki page if you have been provided with an .ovpn file?
https://openwrt.org/docs/guide-user/services/vpn/openvpn/client-luci

What is the openwrt device?

thanks for the reply. i have followed the tutorial and i managed to connect to the remote network. The thing is that i do not have access to the Internet, rather only to the local and remote LAN. The device is an Alix 2d13 from PC_engines (x86)

i have tried many times without success. all of the times, i can access the remote LAN, but not the internet even though i have created the firewall rule.

I presume Windows openvpn client is able to access the internet.

Please provide a trace routes when using openvpn clients for both the Windows client and the openwrt router.

For example:

tracert 8.8.8.8

ps. do both LANs use different subnets? (eg. 192.168.x.x etc)

Windows OpenVpn client can access the Internet and all traffic goes through the tunnel, even for Iinternet access. Traceroute from Windows to 8.8.8.8 goes through the tunel and it works successfully

When having the OpenWRT as client to OpenVPN i can access remote LAN.
Traceroute from Windows to 8.8.8.8 goes through the tunel and it works successfully. I do not have though access to DNS services, ping to www.google.com it does not work, or http/https access to any site.

Local LAN has 192.168.170.0/24, remote network has 192.168.1.0/24
No idea i could fix that.

Is tracert 8.8.8.8 through openvpn client openwrt router successful?

Is DNS correctly configured on the openvpn client router?
https://openwrt.org/docs/guide-user/services/vpn/openvpn/client-luci#troubleshooting

You can ping and traceroute to the Internet by a numeric IP, so the routing is working properly. Need to change the DNS server to one that can be reached through the VPN. The servers operated by ISPs only work from the customer side of the network, so when you route everything through a VPN, the ISP provided DNS no longer works.

by adding DNS forwarders to my machine:

εικόνα
did the trick and i have internet access. The thing is that i have Internet access through the VPN tunnel, not from my local connection (4G, xdsl, etc.). When i do whatismyip i get the public IP of the remote end, not my public IP. How can i resolve that please?

That is called a split tunnel. It can be done by configuring your OpenVPN client to not use the default route pushed by the VPN server. Then you have LAN-LAN access via VPN and Internet direct through your ISP. If you are connecting to a corporate network they usually prefer you don't do split tunnel, so everything goes through their firewall.

no it's not a corporate network, rather as i mentioned already a link between my colleagues network and mine.
how do i do a split tunnel in this situation?

You should configure route. See output of route command. See also https://community.openvpn.net/openvpn/wiki/IgnoreRedirectGateway