First,don't even worry about the HD decoder,there is no HD available for 4DTV anymore.
Once you find a cord for the 920,plug it in,at first the front panel should show all "*****",then change to WMUP.After Warm Up,you will either see a sat and channel number or press the power button and then the channel display will lite up.
Hook it to a TV,you can use either the modulator output(channel 3 or 4 on the tv) or the video/audio outputs (yellow/red/white rca jacks).You don't need to be connected to a dish to check the numbers.
Press the OPTIONS button on the remote,then 6 then 5,the receiver number,line B,should match the the UA number that is on a tag on the back,or maybe the bottom of the receiver.
If the numbers still match the ID is still good and you can buy programming,if the numbers don't match the ID is lost and you can no longer subscribe with that box.
If the ID is bad you can still use the 920 as a dish mover and still receive what free digital (and analog) channels that are still up there.You will have to replace the battery first tho.
If the ID is good you should replace the battery asap,unless you know the bat has been replaced recently,say in the last 2 or 3 years.