... and only have one (1) SSID in the known network list. BSSID lock is only for that network! SFT1200 will jump to other known networks for just no known reasons.
BSSID Lock:
It can be set at "switch network" time, via advanced setting, after selecting the network in "join other network", and it then can be locked into that BSSID. Once done with that BSSID Lock enabled, then the menu option will be available for further modification or to be disabled, for that network only, where it was set.
Yet another extra step could be to set fixed (manual) IP, because that disables the "hotplug" action on the WWAN interface when the connection just disconnects/reconnects. Hotplug action is renew DHCP but also reload Firewall and NAT TCP sessions.
See WWAN in Luci :
"Force link
Set interface properties regardless of the link carrier (If set, carrier sense events do not invoke hotplug handlers)."
Also see other posts: SFT1200 2.4 GHz Repeater Disconnects