I'm just afraid of the failure of the PLM and nowhere to get a replacement, can I plug it to my UPS battery for better protection? thanks!

No not so good. The surge protection in the UPS filters the Insteon signals making them weaker. No harm in trying but you couyend up with devices not responding especially from scene commands. 

Not a good idea to plug the PLM into a UPS as you'll lose the powerline interface. However, if you do plug it into a UPS then you should probably have a dual band device plugged directly into the a/c outlet near the UPS.

It's also a good idea to plug the UPS unto a FilterLinc as the UPS can interfere with the Insteon powerline signals.

 

 

Insteon has said they expect to have new Serial and USB PLMs first quarter 2023:
https://shop.insteon.com/products/plm-serial-modem-interface
https://shop.insteon.com/products/plm-usb-modem-interface

@jzh as suggested above point of service UPS or Surge Protectors tend to block the Insteon traffic and does not reliably allow the signal to reach wireline devices. 

Better option would be whole home surge protection, but still wouldn't entirely protect the point of service location. The new owners of Insteon expect to have PLMs in stock again soon and there are repair services (both by Insteon and found on ebay) in case something should happen to your PLM.

Are you just trying to hedge your bets now or are you having symptoms of questionable PLM behavior? 

Depending on the age of the PLM some of the more recent builds did appear to last longer than prior versions. Don't worry too much about it at this point. The time to worry might have been the last year or more. At this point I think there's plenty of options for repairs and new stock available soon.

 

Revision 2.4 and higher.

Have the new serial port daughter board in then. With a signal protection network on the serial signals. Also have a serial chip with a better ESD protection specification.  A better but not ideal choice. C7 and C13 are now Fujicon TY series caps. Have a better rating for switching supplies. 2.3 has the later serial chip and older serial port board.

I also agree with not through a UPS. X10 and now Insteon power line signals. Are frequently absorbed by the UPS power line input conditioning circuits.  Having the UPS on a Filterlinc can also stop it from absorbing signals on the power line.