The VPN channel is up, on one of the sides there is a TP-Link 842n v3 router with OpenWRT 19.07 firmware as a client.The server is OpenVPN Access Server, was not started by me and the settings were made using databases, there are no usual configuration files about which they write in the documentation.
There is a local network behind the router. Computers from both sides see each other, ping passes. At the same time, the router itself acts as a dhcp server behind the router, but I would like this role to be performed by the main DHCP server, which is located on the other side of the VPN. Has anyone done this himself, and if so, can an example be.
The mode is tun, I cannot change to tap.
Is it so obvious that no one wants to write or is it so difficult?
Difficult and kinda pointless. You'd need a dhcp relay on OpenWrt and the DHCP server must have configuration for another pool.
The idea with tun mode is for each network to independently control its IP addresses with a local DHCP server, then layer 3 route between the networks. The remote network can't have the same IP range as the local one, so it wouldn't work to make requests from a single DHCP server for both. As @trendy said, you'd need to set up the server to control two networks. Which does localize administration but it really isn't much different than having two servers.
Also of course if the Internet or the VPN link goes down, your LAN becomes unusable due to lack of a DHCP service.
please poke into the manual for such a setting or your example