What does Wifi AP TP-Link PharOS "auto" channel/frequency setting?

Auto usually means on a reboot the AP will scan the local environment for interference and move to the channel that it detects will be the clearest from neighboring interference.

These algorithm's used in consumer routers for channel selection don't usually work very well. For permanent links like bridges it should be disabled.

Not sure where you are living or the local regulations there, but channel 100 is a Dynamic Frequency Selection channel here in the U.S.

If you don't have the option to remove auto, that usually means you are on a DFS channel, and there is nothing you can do other than choose a static channel outside of the DFS range for the bridge links.