This is speculation. Typing 6900DTE into the search box at
www.funai-corp.com/support/ shows that this ATSC tuner is made by Funai. Funai also makes a lot of Magnavox stuff, including the TB100MW9 DTV Digital to Analog Converter. The Magnavox manual says for universal remotes if Magnavox codes don't work, try Sylvania codes.
We know the IR protocol for the TB100MW9 is Emerson device 42, which isn't available in the 6131. The UEI setup code is Cable 1563 and it is available in:
*Radio Shack 15-100, 133,134,135 None of these are still produced, but may still be in stores.
*Atlas 5 3000 (Some cable vendors--but lots of Atlas remotes don't have 1563)
*Vizio VUR8 (if you have a high end Vizio)
*OneForAll URC-8820N or 10820N (must be sure this is the newer N suffix, and not B00. Available on the internet)
*RCA RCRP05B (Best Buy $20)
The RCA is probably the easiest to get, but the OFA x820N are also good remotes. I can't guarentee that cable 1563 will control the Sylvania, but I think it is a pretty good bet.