The router can only work at 90mbps speed （RBK853-100CNS）

My router Orbi AX6000 RBK853 has just been bought for a few days, and it was updated with the latest software package during installation. The app tested the connection speed at 900mbps, and everything was fine including satellite. After that, the configuration and position have not been changed. I was very satisfied.

But yesterday afternoon, the app test suddenly dropped to 90mbps, even at the place closest to the router, it was 90mbps. I immediately tested the network speed of the modem port and the result was 900mbps. I restarted the modem and orbi, the router still only has 90mbps. I replaced the old tplink router in Orbi, and the test was 9000mbps. I reset the orbi router and reconfigured it on the app, but it still doesn’t work. What’s the reason?

I think it’s not an upgrade, after all, it’s been a matter of days.
It is not network access, because the tplink router is good.

Thanks

Steve