Hello,
Since the carrier uses 5G NSA configuration, it may be hard to force the device to stay on 5G. However, what you can do, is lock the device onto 4G cells that have 5G availability and hope that the device stays connected to 5G. Run the command gsmctl -A 'AT+QCAINFO' when you are connected to the correct cells, and note the EARFCN and PCI of the LTE cells.
Then remove all of the band locks, but leave the service mode on 4G+5G. Run this command with the number of B7 cells, their EARFCN and PCI information filled in:
+QNWLOCK: "common/4g",<num of cells>,[[<EARFCN>,<PCI>],...]
An example could look like so:
gsmctl -A '+QNWLOCK: "common/4g",2,6400,354,6400,348'
Let me know if this helps!