About 5 weeks ago - I upgraded my Archer C7 (v2) to the current firmware (3.15.3 Build 180114 Rel.39265n).   I also realized during the C7 firmware upgrade that my TP-LINK  RE-450 (v1) extender also had outdated firmware - so I upgraded that as well (1.0.0 Build 20171215 Rel. 55534) - not too long after the firmware upgrades I realized the RE450 was NOT connecting to the router on the 5ghz link - after much wasted time with TP-LINK phone support -  I found that when I turned the C7's  MAC filter off - the 5ghz link would connect between the router & extender.  This does not duplicate with the 2.4ghz link - it will connect whether MAC filtering is enabled, or not.

Additionally, if 5ghz MAC filtering is enabled on the C7 - any device (Android, Apple, Windows) can connect directly to the C7 and the same for the RE450 if its MAC filtering is enabled (whitelist) - however, the RE450 & C7 will not make a 5ghz connection. So I bought a Netgear Nighthawk AC2200 range extender to test - and it connected perfectly to the C7 with MAC filtering enabled (or disabled) thus it appears there is a firmware conflict  between the RE450 and  C7 when used in conjunction and MAC filtering enabled (only on 5ghz).