I am attaching the code if the community see something wrong that helps me.

from network import LoRa
import socket
import binascii
import struct
import time

Initialize LoRa in LORAWAN mode.

lora = LoRa(mode=LoRa.LORAWAN, public=True, adr=True)

create an ABP authentication params

dev_addr = struct.unpack(">l", binascii.unhexlify('01 0E 1A DD'.replace(' ','')))[0]
nwk_swkey = binascii.unhexlify('47400CAAE9B7B6E2A3EDD26F45274BBE'.replace(' ',''))
app_swkey = binascii.unhexlify('D06604A453F36920881C5AD78FE10E4D'.replace(' ',''))

set the channels

#lora.add_channel(0, frequency=902300000, dr_min=0, dr_max=4)
#lora.add_channel(1, frequency=902500000, dr_min=0, dr_max=4)
#lora.add_channel(2, frequency=902700000, dr_min=0, dr_max=4)
#lora.add_channel(3, frequency=902900000, dr_min=0, dr_max=4)
#lora.add_channel(4, frequency=903100000, dr_min=0, dr_max=4)
#lora.add_channel(5, frequency=903300000, dr_min=0, dr_max=4)
#lora.add_channel(6, frequency=903500000, dr_min=0, dr_max=4)
#lora.add_channel(7, frequency=903700000, dr_min=0, dr_max=4)

lora.add_channel(0, frequency=902300000, dr_min=0, dr_max=3)

remove all the non-default channels

for i in range(1, 15):
lora.remove_channel(i)

join a network using ABP (Activation By Personalization)

lora.join(activation=LoRa.ABP, auth=(dev_addr, nwk_swkey, app_swkey))

create a LoRa socket

s = socket.socket(socket.AF_LORA, socket.SOCK_RAW)

set the LoRaWAN data rate

s.setsockopt(socket.SOL_LORA, socket.SO_DR, 3)

make the socket blocking

s.setblocking(False)

for i in range (200):
print("Sending..")
s.send(b'PKT #' + bytes([i]))
time.sleep(4)

rx = s.recv(256)
if rx:
    print(rx)
time.sleep(6)

for i in range(1, 15):
lora.remove_channel(i)

Are you using a single channel gateway (for example another LoPy as as a nano gateway)?

If so you need to remove_channels up to 72, not just 15.