I just bought a brand new SENA 50S at 5/2/2021. When I tried to connect it to my iPhone 11 Pro Max, it connected via Bluetooth. Once I tried to play audio through it, the audio was cracking and breaking up, as only audio playing every few seconds.

I contacted the Sena helpdesk that suggested it might be something to do with the fittings and suggested factory reset, but nothing worked.
Then I managed finally to connect the Sena 50S to the WIFI via the dongle that came with it. Once I had done that I managed to update the firmware to version v.1.07. and the audio streaming worked fine afterwards.

I realized I had upgraded my iPhone 11 Pro Max at 4/2/201 to software version 1.4 (18D52).

There are incompatibility issues between the old Sena 50S firmware(s) and this new version of the Apple software (at least for iPhone 11 Max Pro).

So if you have any similar issues, try this out.