Hello,
For the 5G itself, it should be possible to blacklist the n28 in the WebUI. Navigate to Network → Mobile → General Settings, select the network type as 4G+5G, select all 4G bands and only the n78 5G band. This will force the router to only connect to n78. However, as a side effect, if the signal conditions worsen, you could lose 5G altogether, as the device will not connect to n28 (this band is intended for better coverage).
The QNWLOCK command is useful when locking onto a certain 4G cell. For example, you could try locking onto LTE B1 cell, which could increase you download and especially upload speeds. The exact command for your location would be:
gsmctl -A 'AT+QNWLOCK="common/4g",1,300,372'
This will keep the B20 as the secondary band. It should be noted, that the signal parameters for both n78 and B1 are not great, so locking onto these cells could cause network instability. If you notice any instances of high jitter or ping, I’d recommend at least removing the cell lock for LTE.
Another thing to keep in mind, is that cell lock (not band lock) is removed with every reboot of the internal modem. This is why a custom script was utilized here: RUTX50 - Locking 5G-NSA CA bands manually - #9 by bifteki
Let me know if there are any further questions!
Best regards,