as its in your house you are perfectly entitled to check.
sometimes the splitter inside box only has a tv/data & fm connection, so open cover to check you are connecting the cable to correct one
if tv/data x2 ,connect stb to one , modem to the other
to test, just connect the stb to the tv/data & power up, if stb display shows ch number,, you have saved £75 my friend
ps make sure all connections are tight