Can't connect to main network after adding an access point

[phuwanut2002]

First the setup.
I have a turbospeed TCG220-T as my main router combo thing, it connects to the ISP's cable, have lan ports and broadcast wifi.

I added another router, a TP-LINK TD-W8961ND v3.1 as an access point?

I don't know what to call it, i set up it like this:
Main router SSID: home12/34
AP router SSID:home12/34-ext

But with the amount of people and devices in my house the main router would just restart for no reason every 3 mins or so which is really annoying.(Overheating?)

So it worked for a while, but now suddenly the second router is doing kinda the same thing, my S9+ constantly kicks me off the secondary network and the main network just isn't connectable (is that a word?) by any device whatsoever...
So now what?
What could be the problem?

 

[kanewolf]

Did you follow the steps in the sticky for using a router as an AP -- http://www.tomshardware.com/forum/36406-43-convert-wireless-router-wireless-access-point ?