SNOM dect M65 register to wrong extensions once they got connected to M700 bases

we had some network issues hence the base stations M700 would blink red and loose connection. 
once we reset or reconfigure the bases, they will be back online but the DECT phones M65 which are already registered to those bases' re-register by themselves, but they won't register again to the same extension of the 3CX portal.
There will be always swap with the extensions and IPEI. Is there a way where they register back to same extensions as before.