You might try sending Volume Up as code 123 as this is what is described as working here from the "AB Shutter 3" device (which I've used in the past and can affirm it's worked.)

https://shkspr.mobi/blog/2016/02/cheap-bluetooth-buttons-and-linux/

"It turns out that the iOS buttons sends Volume Up (key 123) whereas the Android button sends Enter (key 36)."

Please let us know if that fixed anything!