Hi,
Probably, the problem is related to
https://osmocom.org/issues/1458
In short, a phone is unable to sync a strong BTS due to the ADC
input overload.
As a quick solution, you can use the 'stick ARFCN' option in
'mobile.cfg'.
This helped me when a phone was pretty close to my BTS.
With best regards,
Vadim Yanitskiy.