Mac address filtering has nothing to do with what I need whatsoever.
What is the point doing this? Well it has multiple reasons:
1, The ISP is so dumb/cheap that they gave me a GPON with 100mbit ethernet port when I have 300mbit/s fiber service so the router can't even serve the network properly so I bought my own
2, If the modem breaks it could take many days for them replacing it, so I prefer using my own modem instead of the ISP's and only keep theirs in case if my would break (at which I would order a new one immediately and would only use the ISP modem as longs as I need).
So no solution yet...