@tomhawkson1990s 

What is the current network diagram? How do you connect these devices together? 

Cause you switched the Deco to AP mode, is there another router existed?

What is the light status of the Deco M4 when you lost connection? Is there any error message on the Deco app? 

In the meantime, please connect your computer to the Deco M4 and test whether you can get internet by wired cable and click here to verify its IP address and gateway. 

And can you get internet from the main router by wired cable? 

Good day. 

@tomhawkson1990s 

Is modem a pure modem or is it a modem router? Cause you switched the Deco to AP mode, as my knowledge, the modem should be a modem router which you can get internet from it directly; and there is no conflict between the router mode and AP mode of Deco units; 

In a local network, only one router is needed, so even though you switched them back to router mode, theoretically, there is only one Deco works as the router, which is the main unit which connects to the modem directly, the slave Decos are still working as the access point or extender( they still build up the mesh network together). 

So if you have switched them back to router mode, you can take more time  to test the performance. 

If they work in AP mode, you can verify the details mentioned on my last reply to narrow down the issue; Besides, you can switch the location of the Deco 2 and Deco 3 in AP mode. 

Best regards.