Lately there have been a few reports of *somethiing" apparently getting “sticky” and solved by a reboot, but I cannot seem to find a “pattern”, in your case it could be DHCP, yesterday in another case it was the LTE (or the APN):
http://forum.mikrotik.com/t/sxt-lte6-kit-lte-problem/183410/1
the day before it was ping/routes:
http://forum.mikrotik.com/t/unable-to-ping-internet-via-isp-router/183409/1