Finally, a solution!
I have just had this response from Mikrotik support:
Hello,
Thank you for contacting MikroTik Support.
FG621 has a different cell lock commands, which are not uploaded to our Wiki, but will be in a short period of time.
In your case, the command will be:
interface lte/at-chat lte1 input="AT+GTCELLLOCK=1,0,0,6175,176"
AT+GTCELLLOCK=<mode>[,<rat>,<type>,<earfcn>[,<PCI>]]
< mode >: integer type; 0 Disable this function 1 Enable this function 2 Add new cell to be locked
<rat>: integer type; 0 LTE 1 WCDMA
<type>: integer type; 0 Lock PCI 1 Lock frequency
Best regards,
Aleksis Graudužis
Thanks so much, again, to Norfok&Signal and MrTwinky2 for helping me to get there!
MrTwinky2 your solution 2 was indeed correct! I now see I missed out "GT" when I first tried - hence the error!
Hopefully this information will prove useful to anyone else who tries to cell lock a Chateau LTE6