AC1750 Netgear R6400 slow internet speed

[Lilibet]

When I setup my new router I now have two networks(not on purpose I am a bit clueless) My wireless connections to devices are slow and drop frequently. I did update the firmware which helped for a while but now slow again. Would getting rid of one of the networks help? Or should I get a wireless extender? I have plaster walls, router on first floor and computer on second floor.

[plemans]

So we need a bit more infomation. 

You have the R6400?

What firmware is on it? 

What modem/gateway is it connected to? 

When you say "you have 2 networks" are you talking about the 2.4ghz and the 5ghz network or does your gateway also have a network as well? 

FYI 2.4ghz is a slow network. It won't be nearly as fast as 5ghz and is more sensitive to interference.