I installed 6.18 onto several netbox 5Ghz. Before the upgrade, the links were rock solid. Since upgrading, the radio units configured in station wds mode locked up and although they were accessible via the wlan port, the ether ports were not. A simple reboot worked but they all locked up again after 6-7 hours. I downgraded back to 6.12 and the problem has been solved.
Netbox 5 and v6.12 - are you sure ? There is no wireless-fp package in v6.12 and it is needed for .ac
I did not have such problems with v6.18, but currently I am using v6.19rc12, give it a try.