CANNOT GET ORPHAN CORDLESS ATT EL52103-52503 DECT 6.0 TO REGISTER WITH BASE

I obtained an orphaned ATT Cordless Dect 6.0 series EL52103-EL52503 phone and cannot get it to register with my base that serves three other of the same phones.  All online instructions say to start with different symbols, etc., that do not appear on my phone.  How can I get my phone to register.  I even unregistered my other three phones, just to try and get them all to register, and the orphan keeps syaing "Out of range or ...").