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FortiGate devices with a 5G SIM card and LTE modem are commonly used for remote and backup connectivity. In some cases, connectivity issues may occur that impact modem operation.
Scenario where 3rd party antennas are in use:
In environments with limited signal coverage, 5G connectivity can be affected even when external antennas are used. While third-party antennas may improve signal strength, they do not always guarantee the signal quality required for stable 5G operation.
5G connectivity on FortiGate depends on having both a strong signal and good signal quality. In locations where radio coverage is limited or where external antennas do not fully support the carrier’s active frequencies, the modem may not be able to complete the 5G network registration.
Under these conditions, the modem may repeatedly attempt to connect to the 5G network without successfully establishing a data session, which can result in the WWAN interface not receiving an IP address.
In this scenario following troubleshooting should be performed:
- Run this command:
diagnose sys 5g-modem modem-details
--------------------
Modem detailed information:
--------------------
Manufacturer: xxxxx
Model: FN99xxxxxx
IMEI: 35678xxxxx
MEID: 35678xxxx
Hardware revision: 1.00
Software revision: M0R.13xxxx
Carrier config: xxxxxxx
Carrier config PRI revision: 5xxxx
Modem temperature: 31 C
Modem operation mode: ONLINE
Modem network mode: 3G|4G|5G|
If the Modem is ONLINE, continue to step 2. If it is not online, follow the related article at the end.
-
Enable LTE Debugging is needed. To gather more detailed information for troubleshooting, enable LTE debugging using the following commands:
diagnose debug disable
diagnose debug reset
diagnose debug console time enable
diagnose debug application lted -1
diagnose debug application nr5gd -1
diagnose debug enable
..........
2025-10-31 15:19:55 [MODEM1]check_RF_and_SIM_ready(379)-1611: Modem1_SM Check modem mode(ONLINE) [PASS]
2025-10-31 15:19:55 [MODEM1]check_RF_and_SIM_ready(387)-1611: Modem1_SM Check SIM present status(SIM_CARD_PRESENT) [PASS]
2025-10-31 15:19:55 [MODEM1]check_RF_and_SIM_ready(394)-1611: Modem1_SM Check SIM pin status(Ready) [PASS]
2025-10-31 15:19:55 [MODEM1]ST_modem_signal_searching(448)-1611: Modem1_SM:[ST_MODEM_INITED] -> {[ST_MODEM_SIGNAL_SEARCHING]} e
vent_generated:0
Based on the debug output, all processes appear to be operating normally, but there is no signal. If the same behavior is observed, the next steps should be tested.
-
Change the network-type to 4G only.
config system 5g-modem
config modem1
set network-type 4g <---------- Change from 4g|5g to 4g only.
set sim1-data-plan "Profile 1"
set sim2-data-plan "Profile 1"
set carrier-config manual
config sim-switch
set by-sim-state disable
set by-connection-state disable
end
end
end
Reboot the modem once this is changed.
Note: If third-party antennas are being used, include this information in the new ticket. Third-party antennas are widely used and supported from a functionality standpoint, but any issues related to RF performance, signal quality, or antenna characteristics are considered environmental or third-party factors, not hardware defects of the modem.
To check whether the modem itself is causing the issue, move the device to an outdoor location or close to a window with clear signal coverage and use the original antennas provided with the modem in the box. The steps described above should also be tested as part of the troubleshooting process. Switching to 4G can still be an option.
If the behavior continues, open a support ticket with Fortinet TAC for further investigation and collect the logs from the related article below.
Related article:
Technical Tip: Troubleshooting 5G SIM cards and LTE modem issues on FortiGate using CLI commands
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