https://community.cisco.com/t5/small-business-routers/rv130w-wifi-slow-down-in-every-2-or-3-days/td-p/2704436
First off, use Wireless Channel 1, 6, or 11. Using channel 9 can cause "overlap" with neighboring wireless networks around you, which is bad. Using channel 1, 6, or 11, allows for co-channel existence and "airtime fairness". In other words, nearby wireless networks will all cooperate and transmit at better speeds. More info here: Why Channels 1, 6, and 11.
2. You should also try Wireless Mode: N-Only. Using B/G/N-Mixed can slow down wireless N devices because you may have to dedicate some time to B/G airwaves.
3. Lastly, for your Wireless Band Selection, try just using 20 MHz. Using 40 MHz with a 2.4 GHz radio is not recommended in dense residential areas.
