Welcome to the community! I would be happy to help you get your rear speakers set up.
When trying to set up the rear speakers, at what point are you having trouble? Is the left speaker not found during the setup or does the app find the left speaker but not stay connected?
The first thing to check when only one speaker will not connect is the Left and Right switches on the receivers. You need to make sure one is set to 'L' and one is set to 'R' for them both to connect.
It is also worth looking at the location of the left speaker. You do need to check whether there is anything that could interfere with or block the wireless signal to its location. If you have any wireless devices near your soundbar or rear speakers, such as a router or wireless printer, these can interfere with the signal. It can help to move the speaker closer to the soundbar to complete the connection process.