Technicolor DPC3216 Modem to Netgear AC1900, R7000 router 80 feet apart not connecting

> My router (6 mths old) is not connecting once I move it 80 feet from
> the modem. [...]

   Are you talking about the distance or the cable length?

> [...] I have two cables that I have attached ends to, and tested both
> cables with an ethernet cable integrity tool (continuity?). The cables
> test fine, but do not allow the router to connect to the internet.

   These cables are made from what, exactly?  (Category what?)  Does a
shorter made-by-you cable do better?  What do the LED indicators on
these devices indicate?  For a test, using a LAN port on the R7000
instead of its WAN/Internet port might tell you more about the cable
(link rate).

> [...] I have an AC1750 router (in an access point configuration) 300
> feet away in a boathouse. [...]

   100m is the value usually given for the max cable length, so that one
would be close to the limit.