Second Router installation in external Garage

I tried that but still nothing

Do you get online if you connect a PC to the ethernet in the garage? Maybe it is terminated wrong. Are you using the WAN or LAN port on the router?

I have a external wavelink antenna acting as a AP and it works when connected to the cable.....

If one AP works then another AP should work. Make sure you change the IP of the router to 192.168.1.1 or anything that doesn't conflict with the Cox gateway's 192.168.0.1 IP. Also, try a power cycle of the gateway. Shouldn't be required but a good troubleshooting step to try.

No problem! I pulled up the manual and seems the default IP for that model is 192.168.1.1 already so there should be no conflict. Make sure you are using the WAN port on the router and then check if it is getting a WAN IP using the Netgear app.

I don't see what. The Cox router should see it just like any other device. You don't have the gateway in bridge mode do you? I will be sure to check back tomorrow morning.

The only thing I can think that would help is adding a DHCP reservation for the AP's IP however that doesn't seem possible with the Cox Panoramic gateway. That might explain why it worked for a bit then stopped.

The router the OP mentions has a AP operation mode. See instructions in the first link I gave. As long as you are using DHCP you shouldn't need any routing/subnet knowledge. Can you explain? Are you basically saying it is user error?

If the nitehawk is setup as an access point, there should be no issues. The nitehawk will pass through ALL dhcp requests to the pano modem If another device is working on that ethernet wire, then try doing a factory reset on your new router, then login and setup as an Access Point, BEFORE connecting it to the ethernet. 

Good advice. I am going to assume OP got it working since they never posted back.

The idea of AP mode will differ from one vendor to another as how this is not any kind of industry standard.  Router are router and AP's are AP's but telling the result of telling a router to be an AP will differ by manufacturer.  Given what the OP is describing, there's clearly some other underlying issue occurring here.  My advised is based on eliminating a later to the issue that may not otherwise be necessary.

I think it depends on the device and configuration. I know on my Asus RT-AC1900P in AP mode I am using the WAN port. There are actually two ways to run a router as a AP and I think your talking about the old-school method. See discussion here for more info.

PS. Thanks for helping out here on the forums. Not many stick around after posting their problems. Welcome!