I've had this wireless router for a couple of years now and haven't had any issues, but I recently upgraded my internet service from 200mbit down/10mbit up to 940/38 (Spectrum internet). The current setup is cable modem -> Archer C5400 -> 2.5GbE unmanaged switch that connects to a number of other machines and another 2.5GbE unmanaged switch which goes to a few other machines and a printer. Nearly all of the time, only 1 PC and 1 printer in sleep mode is actually powered on on the network. Theres a small handful of wifi devices too but most remain powered off, maybe 1 or 2 run netflix. No consoles/games.

The problem is the downstream speed eventually dwindles to ~200-300mbps instead of the advertised 940mbps. By eventually, it may take a day to a couple of days for it to suddenly lose speed. I can initiate a remote reboot of the router periodically and it restores the speed back to 940. I do not share the wifi password, have a reasonably secure password, and have even disabled wifi and direct connected only 1 strong computer to the C5400 router, results remain the same. I've tried rebooting the cable modem and sometimes it does not fix the speed, but rebooting the archer c5400 always fixes it.

There are no crazy settings enabled and no port forwarding. Just the typical 2.4ghz and 2x 5ghz wireless bands enabled, and DMZ set to the strong computer's IP address (since I'm too lazy to deal with some port forwarding)

What can I do to make sure it doesn't drop speed in the future without resorting to regular reboots on the router?