The answer to your basic question is yes, but in order to do so, the computers have to be on the same network and it isn't at all clear that this is the case for your setup.
The issue is not download speed or what wifi band you're using.
It sounds as if you have two routers. That will definitely cause problems.
Open a Command Prompt window on each computer and run the following command:
ipconfig
What is the IP address and Default Gateway for each computer?
Please describe the signal path from the Windows 7 computer to the Windows 10 computer.
As I understand it, "DECA" is a coax to Ethernet adapter. Thus:
- Windows 7 to router A via wifi
- router A to DECA unit via Ethernet
- DECA unit to Internet via Direct TV coax connection
- Windows 10 to modem/router B via Ethernet
- Modem/router B to Internet via Direct TV