Update here:
Problem solved!
Last friday we applyed iPerf test set from phone to internet gateway (PGW).
UDP tests were successful, but TCP tests failed.
Due to heterogeneous backhaul the BTS would guess backhaul failures were congestion and started to apply QoS.
Once every packet from CORE came with Best Effort BTS put it into low QoS and restricted resources to applications such as http.
We changed QoS factors for best effort and activated a BTS feature + reduced MAX PDU. It worked!
Thanks for all suggestions!