The DECT repeater does not support any other access code different to 0000.
If you want to add a repeater to your M300,M700,M900 DECT base, you need to configure AC code to 0000.
1. Change code to AC 0000 as shown.
2. Add new repeater and do engage procedure
Once it is done you will see your repeater correctly paired to the DECT system.
Once the repeater is registered, you can change your Access Code back to the previous value.