This is an old story, the 5Ghz link is own after a few days of use. There is a thread where people are suggesting to downgrade to 1.14. I personally have the latest 1.17 that fixes the KRACK security issue in a couple of these APs, but It seems that this firmware has the same bug that was introduced in version 1.16.
Could you please release a new firmware that includes all the krack security issues and also that fixes once for all this 5Ghz link issue?
Thanks.