It probably is but the link frequency may be different! Your Avant will be sending the signal and at a guess the LX is controlling the box but the channel used for the device that you are calling up is not the correct one. Usually the Avant uses a Link Frequency of 599. It depends what the Tape and Sat are receiving on the LX as to whether you get a picture. I cannot remember which channel is which as the LX5500 is not a set I have had! The channels are usually 61,62,63,64 etc. You need to either:
1. Download the Product Configuration Guide from here - you will need to be a Silver or Gold member.
2. Get your dealer to run off a copy of the configuration for your system and it will give you the set up instructions. I would do this for you but I have a Mac and the software doesn't work!!! Windows only.
That will explain how to tune the channels in and which ones to do.