This depends on the base station that your router is connected to. If the carrier in the base station is not capable of SA 5G technology you won’t be able to setup 5G only.
However, you can overcome this with manual intervention.
thanks for the information, that explains the observed behavior. But the solution is somewhat finicky, it would be really neat if the firmware could get some sort of 5G preferred feature to solve this issue without resorting to scripting and/or do location specific configuration.