Share wifi connection from macbook to airport express

The DHCP works fine for the airport.

If you use ethernet and a static IP address, then you'll have to choose an IP address outside of the DHCP lease range otherwise you can get IP address conflicts.

For example, if your DHCP range is set to 192.168.0.10 through 192.168.0.20 then you can use the IP address ranges 192.168.0.1 - 192.168.0.9 and 192.168.0.21 - 192.168.0.253 for statically assigned IP address.

Hope this helps

You're connecting a router to a router, so you will get dual NAT.

The shared router is translating the single IP from the ISP into a shared network for you all to use. If you connect a second router to this to connect multiple devices, then you are creating a second network, and network address translation (NAT) is happening twice.

I'm not sire if the airport express supports bridged mode, but you could try this to see if it will allow you to connect multiple devices over Wi-Fi with the main shared router distributing the IP addresses. If you can do this, it should solve your dual NAT problem.

Dual NAT shouldn't however cause intermittent connection, that's more likely interference from other nearby Wi-Fi devices, so I'd suggest selecting different channels (if possible) until you find one that gives you a more stable connection.