TP Link Archer C3150 setup as an access point and switch help

Hello all.

I bought two TP Link C3150 routers and are trying to use one as the main router and the second one as an access point plus switch.

The reason for the 2nd router to also be a switch besides being an access point is because the main router is at the back end of our store mainly connected with a DVR security system. The 2nd router connects to the main router and then connects to the reception is computer and also acts as a n access point to provide an extra area for wifi connections.

The problem is, all the guides I researched just shows how to make the second router an AP. If I disable the DHCP server on the second router, will the reception computer still be able to gain internet connection via the second router?

I use DMZ on the first router to open our IP for public access to our DVR security cam system. However, when I connected the second router with DHCP on, both my receptionists computer and the wifi of the second router are working. But the DVR is not working. I suspected it's because both routers are using the same range of IPs for the DHCP servers respectively.

I tried to change the range of the second router but got an error code 85011 "please input end up address". I have no idea why this is happening. I switched off the DHCP server for the second router and magically, the receptionist computer still was able to connect to the internet. But my DVR is still not working.

I hope I provided enough information on my situation and I hope some one could help. Thanks!

Update: weirdly, I tried resetting both the routers, setup the first router again and now the DVR is working. I plugged the lan cable from the first router to the second router's WAN port instead of LAN port and the second router automatically detects the settings of the first router and matched the settings including the wifi SSIDs and passwords, as well as login password. Should I now change the IP address of the second router as instructed from the guide to a number outside the DHCP range of the first router?

Also seems like the DVR not working is due to address reservation. Now with address reservation off, the DVR is working again lol. Should I use IP Mac binding instead of address reservation in this case?