In this case, it would be best to set the LAN IP of the TRB245 device and update the firmware without keeping the settings. It will conflict with another gateway for a short time, but if you set the LAN IP through SMS utilities you should get both gateways running in no time.
To change LAN IP address through SMS utilities we will need to use uci commands.
While you still have your device untouched, go to CLI and check what uci line is for LAN IP, you can do that with command
uci show | grep lan
And you should find something like:
So now when we know what command looks like, just update your firmware without keeping the settings and once it's done execute these commands within SMS utilities, just use the WebUI password and add these lines into SMS, but make sure they're sent separately starting with WebUI password each:
uci set network.lan.ipaddr='192.168.2.1'
After this, you should regain your connection to both gateways in a couple of minutes, unless you need to set something in your TRB245 to reach it as you reach it now, then you can also do it through SMS utilities and uci.