RT-N13U Rev 1B as access point hangs network

[gavinrs]

Have an Asus RT-N56U router which works great. A few months ago I attached N13U as Access point. The 13U is connected via an Ethernet cable with a fixed IP address. All works fine for a day or so and the appears to hang the network. I power down the AP router and reboot the cable modem and M56U router and all works fine.
Questions.
1. Any ideas how to fix?
2. What is best router to try as an Access Point?
3. Should I purchase a new router with improved range - which one?

Thanks so much.
Gavin

 

[L&LD]

Is the fixed IP address outside of the DHCP range of the main router?