Router keeps capping itself to 100Mbps

Hello! I have a 1Gbps connection from my ISP and my router has been working fine for about 6 months with delivering this speed. But recently I've found that the router caps itself to 100Mbps within a 24hour interval. Whenever I reboot/clean install the router it works as expected and gives out 1Gbps, but after some hours/next day its capped at 100Mbps again...

Any suggestions? Thanks in advance