In the second topology, your main network's IP is 192.168.1.* but clients connected to WR941ND show 192.168.0.*. If DHCP has been disabled on WR941ND, how is it possible? Besides, it is normal that...

C7 is a router and not many people buy it but use it as an access point...But anyway, it is working quite good in my case. If you verified that it is a configuration or firmware problem or something...

When the router works as an access point with DHCP disabled, it is transparent and has no NAT. Why you still configure DDNS on it? Sounds strange. Your NAT router has no such a feature?

Have you tried different wireless clients? How about Wi-Fi signal strength? Try different channels and channel width 40MHz. Ensure your network is secured with WPA2-PSK, AES. Check the speed showing...

In sequence? What does it mean?

Good luck with a new router. :D