This is straightforward. The separate DHCP server needs to be configured to advertise the OpenWrt router's IP on the LAN as the network's default route to the Internet (option 3) and usually also the DNS server (option 6). Then the hosts that have a request for the Internet send it to the OpenWrt router, which NATs and forwards it in the usual way to its LTE WAN interface. For this to work, the OpenWrt router needs a known constant LAN IP, that is usually done with a DHCP reservation.