While cellular client devices are common within OpenWrt (i.e. a wireless cellular wan), running a 5G/cellular mobile broadband network requires specialized equipment that is not really in the domain of OpenWrt.
OpenWrt is a router OS which can certainly serve at the core of your IP-level network if you want, but it's not going to give you the operator level cellular radio systems you need.
Can you draw a diagram of the system topology you are considering and how and where OpenWrt fits in?
EDIT: It's also worth noting that in most, if not all jurisdictions, you'll likely need a license to operate a cellular network of any kind. And towers or buildings upon which to mount the antennas. And the cellular broadcast equipment necessary to setup a network. And a whole lot more, normally. So if you haven't gotten that far yet, you may want to start with that understanding the requriements for building out a network from regulatory considerations to logistics to equipment required. OpenWrt would probably only play a small part in your network.