Along with furnace dyeing, water softener dyeing same day, Tuxedo Keypad dyeing same time, my eisy isn't working. After dealing with all the other issues now getting a moment to address the eisy. Upgraded my ISY-994i to the eisy six months ago and used my existing serial PLM and used the serial adapter cable. System has been working fine. Noticed none of my landscape lighting has been working which was the trigger to check out the eisy. Using the LAN admin Console I cannot communicate with any of my Insteon modules so did the diagnostic PLM Info/Status and get the "Not Connected" error. The 2413S PLM has a solid green light, I've unplugged and plugged in several times. Unplugged the eisy several times still no communication.

If I need to replace the PLM is there any benefit to go with USB module or the Serial since I already have the serial cable?

Not sure is it's just a coincidence but had a power failure at my house and power out about four hours before power was restored. Next morning water softener is locked in permanent recharge cycle at 4:30 am and running for hours. House is 65 degrees. Didn't notice the Tuxedo Keypad being dead until later. Same with the eisy. Could just be weird timing and coincidence.

Sounds like a power blip that might have been worse than it looked.

You could try a factory reset of the PLM. If it then shows as connected, then do a PLM restore.

If you still have the ISY994i kickibg around, you could try connecting the PLM to it and see if it shows up as connected in admin console.