Hi, I am using the p8 watch as a dash for electric unicycles, not very similar to the bangle.js, no gps or compass, and a monopole for the antenna, not very precise in shape or length like Fanoush said . I was too having signal problems when in open space (less reflections, more environment/motor noise).
The p8 has good bt signal, some 10 meters indoors, yet I wanted more to keep a solid connection outdoors, so I played a bit with the monopole length. It is possible to get it to a phone level signal, the best way is trial and error, start with the longest length you can fit in a nice shape and trim off mm by mm, till you get the results you need.
I looked at the disassembly video of the bangle.js. and I cannot see the bt antenna.
I also see it has a metal bottom cap, this may explain the bt range problems. I thing it is worth the trouble to try and find the antenna, and maybe change it to a monopole and move it to the front somehow, it may help others too.
this is a p8 in action :)