Create a group in Phoscon, add your Light(s) to that group. Get the ID of the group - lets say its ID 161. Convert 161 (decimal) to HEX: https://coolconversion.com/math/binary-octal-hexa-decimal/Convert_decimal_number_161_in_hexadecimal_ - its 0xA1. Open deCONZ GUI, Open the Bind Dropbox. Drag and Drop your On/Off Remote to SOURCE. Select Group at destication. Enter 0xA1. Press bind and now press a button on the remote to wake it up. The binding should success and it should work.
@OMR described how it works here: Trådfri open/close switch with FW 2.3.079 fails to pair with blind - #5 by OMR - but he used Unicast instead of group binding.
See TRÅDFRI remote control (FW 2.3.014) / TRÅDFRI on/off switch (FW 2.2.008) automatic group is not selectable in phoscon · Issue #132 · dresden-elektronik/phoscon-app-beta · GitHub where I was introduced to the procedure