I had one Yamaha BT soundbar some years ago. My memory is not so good, but to re-pair it I remember that I had to:
Forget that device on my iPhone, then turn off BT (on iPhone).
Restart soundbar completely.
Turn on BT on the soundbar's remote control (perhaps a green LED appeared) and that was its discovery mode.
Then turn back on BT on my iPhone, I had to input a PIN code (usually 0000).
Did you perform that (or similar) procedure to re-pair it?
As already suggested, I think you should contact Yamaha support for that.
I do not think it's a problem related to your iPhone, since you told me that even your iPad or your son's iPhone can't connect to that device.
Regards
Giulio