router won't connect to internet unless I connect computer to LAN

[travis14]

Router Firmware: V1.0.0.48

Modem: Spectrum Business - Technicolor ET2251

OS: macOS / IOS

This is at my office and I can't access modem since its locked in a closet somewhere but when I arrive in the morning the internet light is amber... power, wifi, lan are all green.  When I try to connect to website on my phone using wireless it says no internet connection.  As soon as I connect my laptop to the lan, the internet light turns green and both lan/wireless devices can connect fine.  When I disconnect laptop from lan internet light turns back to amber and nothing can connect.  I don't have any unusual settings for router, I normally don't mess with that stuff and everything works fine.  I've tried both 2.4 and 5G bands and doesn't matter.

When in the bad state, I can login to spectrum and it says the modem in connected

When in the bad state, I can connect to router 192.168.1.1 and view router settings

When in the bad state, I can reboot from router webpage and power cycle and the internet light is still amber

As soon as I connect my laptop back on the lan...everything works again.  Is there some setting to have wireless wake up the internet connection? Or some keep-alive type setting so the router is always connected to modem?

[plemans]

what modem is in that closet? 

Who manages it? 

Is there another router on the network or connected to that modem?

[travis14]

this is specturm's modem (Technicolor ET2251) and they manage it.  my office is in a building with other businesses so the closet probably has multiple modems in there, but its a simple modem with coax and ethernet port so nothing else should be connected to the modem itself.  I'm not sure how they manage the network.

[plemans]

I'm more curious if its a shared connection. As it in goes modem--->router---->your office. 

So if they have something else connected to the modem 

[travis14]

I'm not sure.  I don't think my setup is anything unusual.  I'm relatively new to the building but I can't imagine other offices living with this problem.  In fact, I would assume other offices may have only wireless connections to their router/modem setup.  I just don't know why everything works fine if I have just one device connected to LAN.  If its not an easy config setting or something, I may just try a different router.

[plemans]

I'm trying to figure out if you have a double nat that could be causing your issue. 

https://kb.netgear.com/30186/What-is-Double-NAT

If they have a modem + router setup, when you attach yours it could be causing issues.