As of yesterday my 3bb router is only showing the 5G connection. The 2.4G connection that used to run as well as the 5G connection has disappeared. I have rebooted the router a few times, to no avail.

The problem is that the home's solar power inverter only connects to a 2.4G connection, so it has been offline since yesterday.

Is there a manual way to get 2.4G connection back up and running myself, before contacting 3bb ?

You should be able to get into the router settings and see if anything looks amiss. Address (something along the lines of 192.168.1.1) and password should be on the router label, unless 3BB mess with it to keep you out. Mine looks like this.