Is there anything additional that needs to be done to get 5G support enabled?
With or without a SIM inserted, mmcli shows only up to 4G LTE supported:
root@OpenWrt:~# mmcli -m 0
...
-----------------------------
Hardware | manufacturer: Quectel
| model: RM520N-GL
| firmware revision: RM520NGLAAR01A07M4G
| supported: gsm-umts, lte
| current: gsm-umts, lte
...
-----------------------------
Modes | supported: allowed: 2g, 3g, 4g; preferred: none
| current: allowed: 2g, 3g, 4g; preferred: none