Sitecom Gaming Router: Changing Lan IP

My kids have been asking for a gaming router for ages, finally, I caved in, and went out to spend some cash on a gaming router. Sitecom seemed to be a decent choice for the money (€149).
I wanted to install this router in cascade with the main router & modem along with 2 more routers on the network. The default LAN-address for most routers (192.168.0.1 or 192.168.1.1) were already in use, so I needed to change that. Normally a 30 second job.

Using my laptop running Linux, and Firefox, I changed the router's LAN address (192.168.5.1). To my surprise, it didn't work. Each time after changing and applying the new address, the default (192.168.0.1) would come back. The router did NOT save the settings.

Trouble shooting began: firmware update, trying numerous of changes in other settings before applying the new LAN address, Nothing could help. In the end for the sake of it, I tried the same with Chrome. Even more surprised now, as this was indeed just a 30 second job. It worked ! That was a first ever to notice something to work on chrome that didn't on Firefox.

In short, This router seems to not like Firefox, and needs to be programmed with Chrome. (for windows users, Explorer might work too, but I didn't test further)

Below, the video of my findings: