You can use your own router but many forummers here reported that the 4G backup modem will not run on other router other than Maxis stock one.
So it is quite a difficult situation.
So up to you, use your own router but lose 4G backup, or use stock Maxis router yet got 4G backup if fibre is down.
The only thing I know make sure both fibre and 4G data backup do not run at the same time.
If Maxis detects it, it may charge you or block the SIM.