I had tried it on the main outputs. Still did not work.
It shouldn't make a difference, but please try a couple of short Tip-Sleeve cables.
I tried the link you sent to "configure the channels..." and the link did not work. Can you resend.
I'm sorry about the link. I've fixed it now. Configuring Aux Outputs
I tried using the headphone output and it did work, but not an ideal situation.
Unless you need the headphone output for headphones, using the headphone output is the only way to have an independent volume control for the remote speakers.
Swinging back around to that, are you running two remote speakers using two separate receivers?
What cable you are using to connect the T4S headphone output to the Alto transmitter.