Hi there, sorry to hear you're having trouble,
I think you are missing a step here,
you are correct about going into the handset menu then registration, then register handset,
however when the handset is in pairing mode like that, you must then go to the base station (with the new handset close by) while you press and hold the intercom/locator button on the base station for about 5 seconds. Then you go back to the handset and press OK. You'll hear a long confirmation beep indicating the registration completed.
let me know if that helps. If not, please let me know the model number of the new handset (not the base) so I can confirm if they are compatible or not.