Hi,
Zeroshell is very outdated when it comes to wireless. After a lot of tinkering I was able to setup 11.n with 144 Mb/s and 11.ac with 360 Mb/s
You will need a new version of hostpad and a different config plus a patch of current software.
Here is a howto:
https://www.zeroshell.org/forum/viewtopic.php?t=7143