The network components:
Switch (10 clients, 1 server (Windows Server 2008 R2 with AD/DNS/DHCP), 1 router)
Now I bought a new Router (ASUS AC66U) for the second floor, which I configured as Accesspoint so I can use WLAN, also connected to the switch.
I flashed DD-WRT and everything seemed to work fine, but sporadically I lose Internet connectivity on my WLAN connected devices. Later I discovered, that I also lose connection on my clients when that happens.
I ran a wireshark capture on one of the clients and what happens now (reproducible) is that when I open a URL I get lots of TCP DUP ACK and TCP RETRANSMISSION packets, the site opens very slowly, if at all. Now the interesting part: When I turn off the AC66U and open up a site it instantly loads and there are maybe 1-2 retransmission packets.
I didn't run the wireshark trace on other firmwares I tried (ASUSWRT Merlin, stock firmware and TomatoUSB by Shibby) but the symptoms are the same.
In the source of the packets I see the MAC adress of the first router, which connects to my ISP and I guess there is some kind of conflict but I am by no means an expert to network related stuff.
I would greatly appreciate it if someone could guide me through the setup of DD-WRT to coexist peacefully with the other router or give me some useful hints on how to solve this problem.
Thanks! Please let me know if you need more information on the setup.