RBK30 slow download speeds after 2 days of use

[sailtovict]

My download speeds go down from 300 mbps to 90 mbps after 2 days of use. Both wired and wireless connections show the same behavior.  I have about 20 devices connected. After a reboot speeds are back to 300 for 2 days and then it goes down again. When I do the speedtest from inside the router page, it is always 300+, so the throttling is happening after the router.

[FURRYe38]

What FW is currently loaded?
What is the Mfr and model# of the IP modem the NG router is connected too?

What is the size of your home? Sq Ft?
What is the distance between the router and satellite(s)? 30 feet is recommended in between them to begin with depending upon building materials.

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

Try disabling the following and see:
MIMO, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Save settings and reboot the router and satellite(s).