When the TH400 towel heater shows the abbreviation "BP" in the display, the unit is in the so-called bypass mode. This mode is intended for control via an external thermostat. Normal operation via the unit buttons or via smartphone app is not possible in bypass mode.
To exit bypass mode, press the + and - buttons simultaneously. The "BP" display then goes out accordingly.
If the process is repeated, the bypass mode can be activated again.