Linksys E2000 (dd-wrt) DHCP in AP mode

I have configured my Linksys E2000 Router as a switch via dd-wrt.

Devices coected by LAN:

• Samsung Smart TV
• ISP provided IPTV device
• Cable going to my actual Router (ISP provided, also the DHCP server)

WiFi is in AP mode (bridged, with WPA2 security), it should provide access to the inteet over the ISP Router. I have tued DHCP completely off in dd-wrt.

Problem is, WiFi works great - for about 5 minutes or until I do an ipconfig /renew, then it crashes (no device can coect, existing device slowly discoect, Inteet stops working immediately) and only fixes itself after I reboot the router.

I have also tried DHCP forwarder mode. Sadly, I can't tu off DHCP on my ISP router...

And BTW, is there a way to access the web interface of that router? As soon as I coect it to the inteet I loose access to it. Maybe also a DHCP error?