I got it working this way:
HearLink 3-in-1 → Bluetooth (SBC) → cheero TouchBone CHE-628
About pairing
Pairing was a bit tricky.
At first, I tried the Wireless Remote Mic mode. Pairing the two devices was easy, but initially I could only hear sound from the left side. Then, after about 3–5 seconds, the sound disappeared completely.
I thought the HearLink might be defective or not fully charged, so I charged it for about an hour.
After charging, I tried the TV Transmitter mode, but the result was the same as with Wireless Remote Mic mode.
Then I tried pressing the MF button twice quickly. This time, I could hear sound in stereo mode, but the sound was really strange.
I think it might have been caused by a sample rate mismatch or something similar.
However, after about 5 minutes, the sound suddenly returned to normal.