I have Teltonika FMT100 with FW 03.25.07 Rev:00. I have server of my own and I use UDP. See the log text below, sometimes FMT accepts response from server, sometimes not although the response structure is the same.
[2020.03.03 09:01:38]-[NETWORK] VR socket 0, real socket id: 4, attempt automatic data reading for application
[2020.03.03 09:01:38]-[NETWORK] attempting to read data from socked id (real): 4
[2020.03.03 09:01:38]-[NETWORK.PARSER] Parsing data from 0 socket, data len: 8
DUMP DATA START:
01 1C CA FE 01 58 04 00
DUMP DATA END
[2020.03.03 09:01:38]-[NETWORK.PARSER] presumably udp nod received, captured pid: 88, nod: 4, expected pid: 88, nod: 4
[2020.03.03 09:01:38]-[CODEC.APP] 0x00000095 record marked for server:0
[2020.03.03 09:01:38]-[CODEC.APP] 0x00000094 record marked for server:0
[2020.03.03 09:01:38]-[CODEC.APP] 0x00000093 record marked for server:0
[2020.03.03 09:01:38]-[CODEC.APP] 0x00000092 record marked for server:0
[2020.03.03 09:01:38]-[REC.SEND.1] server accepted records
[2020.03.03 09:01:38]-[REC.SEND.1] Min required records was sent
[2020.03.03 09:01:38]-[CODEC.APP] Rec found at 0x00000096, sort:1
[2020.03.03 09:01:38]-[NETWORK] attempting to send data by VR socked id: 4, data len: 91
[2020.03.03 09:01:38]-[NETWORK] [SEND.DATA.0641] Waiting for socket send result, waiting: 10 sec
[2020.03.03 09:01:38]-[NETWORK] data send to socket: 4, successfull! evt sending: enabled, VR socket: 0
[2020.03.03 09:01:38]-[REC.SEND.1] records packed: 1, waiting 30 sec for answer
[2020.03.03 09:01:38]-[REC.SEND.1] link timeout reset for server: 0
[2020.03.03 09:01:39]-[NETWORK] [IND] received data from socket!
[2020.03.03 09:01:39]-[NETWORK] notification, socket id: 4 has data which must be read!
[2020.03.03 09:01:39]-[NETWORK] VR socket 0, real socket id: 4, attempt automatic data reading for application
[2020.03.03 09:01:39]-[NETWORK] attempting to read data from socked id (real): 4
[2020.03.03 09:01:39]-[NETWORK.PARSER] Parsing data from 0 socket, data len: 8
DUMP DATA START:
00 59 CA FE 01 59 01 00
DUMP DATA END
[2020.03.03 09:01:39]-[MovDetect] Delaying movement 1 -> 0, cnt:2 tmo:60
[2020.03.03 09:01:39]-[NETWORK.PARSER] presumably udp nod received, captured pid: 1, nod: 89, expected pid: 89, nod: 1
[2020.03.03 09:01:39]-[NETWORK.PARSER] records rejected
[2020.03.03 09:01:39]-[REC.SEND.1] ERROR @ m2m_task_recsend_1.c(767):server rejected records, rec sending canceled