I have a modem (Zyxel P660R) wireless Router (ASUS RT-AC68R) combination. The default IP address for both units is 192.168.1.1 So I changed the modem address to 192.168.0.1 and can access now both units. Internet works, but really slow and I'm getting a lot of 'server not found' error msgs. I found hints, that the modem needs to be in bridge mode, which I could define. I just wonder, where to define the ISP information in this situation? Clarification would really be appriciated.
Your Zyxel box is a modem/router. If you don't see a configuration option to disable its NAT and DHCP server features to make it just a transparent bridging modem, then it may not be capable of being just a modem. Note that if your ISP requires PPPoA, it probably can't bridge over that kind of link, so you probably can't turn NAT off in that case. If your ISP requires PPPoE, you might be able to disable PPPoE in the Zyxel box, and that might allow it to show you the settings to disable NAT. Then you'd need to configure your ASUS box to do PPPoE and NAT.
If you can't work out a way to make your Zyxel box be just a modem, you could decide to live with it and set your ASUS box to be just a transparent bridging AP.