Depending on where you are from there are probably a few options. Here in America the cheapest way into 5G would be something like a inseego m2000/2100 which run under a $100 used. They can be reprogrammed for a fee on ebay to bypass carrier QoS and speed limits/throttling. They are also older and don’t support latest bands/speeds.

Second suggestion is an iPhone SE (2022) as it is 5G(supporting a large number of bands), esim and regular sim (with dual standby so both active, and data switching so fail-over built-in), and is supported well by openwrt via usb (you just have to make sure you have the packages installed). They retail for $430, but new ones are often $200 with prepaid line deals, and is under $200 on ebay used.

Third option that may be more plug and play for your setup is to take just about any android >= 11 with 5G and USB-C and connect a usb-c to ethernet adapter. I can’t speak to how reliable it is long term but you should be able to just plug in the adapter to the phone and enable ethernet tethering and connect the ethernet portion to your WAN port.

Qotom makes some nice mainboards and hardware that accepts a SIM card, but you’ll have to buy a mini-pcie 5G modem to go with it

You do what now? Yes you can use a 5G connection with it, I don’t believe it has a modem which is what I think you need to replace. Most likely you’d have to call your ISP and register a new modem anyway Sierra Wireless Cradle point Etc. Probably something you’d have to be on a business account to do though. I would buy a cheap 5G Android phone and Root and Magisk then dialup using tether in openwrt