AC1750 R6400v2

Alexa dot devices (4) randomly disconnect and respond "trouble understanding try later" or "looks like router issue".  They reestablsh contact only by turning off power to device and turn back on.  Turning router off/on seems to have no bearing.  Tried different channels, same problem.  When device not operating the network map shows dotted line with the wi fi signal, but only establishes re connection by device off/on.  This problem is frequent.  Other devices occasionally but infrequently disconnect also but need to be restarted via router off/on.  Other devices never seem to have a noticable problem.  Alexa devices have all been reset without change.  Since we use them for personal safety alerts to others it could be a problem if we needed them and they did not respond to voice command. Router is 6 mos old.  Is this router failure than can be corrected?  Is the router defective? Could it be unstable internet (test all show good)? 16 devices are connected.  Distance to router has no bearing.