Searching the forum was not productive. Please forgive me if my question is "something everybody is supposed to know."
Have four PA9020P adapters installed for over a year. Working fine. Today, I realized some devices were off-line and discovered that all four Powerline units are showing RED LED's. The tp-link utility connected to the primary unit will show only 1, then sometimes 2, and sometimes 3 adapters. The fourth one has not appeared (yet). No changes to electrical wiring. No changes to anything that I can think of.
How do I fix this? Should I go around to one after another and perform a "reset" until Powerline magically heals itself? Should I unplug them all and start over?