The DECT repeater doesn't support any other access code different to 0000.
If you want to add a repeater to your M300 M700 DECT base, you need to configure AC code to 0000.
So if you change Access Code to any other, please consider to need to get back this code to default one.
1. First change code to AC 0000 as shown.
2. Add new repeater and do engage procedure
Once its done you will see your repeater correctly paired to DECT system.
Later you can change your AC code to your usual value.
IMPORTANT NOTE: adding a repeater into an M700 multi cell installation is not encouraged and should be avoided.