MONO output from STEREO amp (T-Class)
The TA2021 in the Topping amp is already BTL when driving loudspeakers.

So you best bet for using it in mono is just driving both inputs in parallel and connecting two loudspeakers. If you've only got one 8 ohm loudspeaker you can only get 14W from the TA2021

Two 4 ohm loudspeakers would be best.

Now about that mono interconnection:

If you connect the loudspeaker between the left+ and the right- output you'll get the sound in mono (an even mix of left and right inputs)

with that 8 ohm speaker you'll get up-to 14W