Why do some of my XBee 4G units work and some dont?

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Hello. I have 4 XBee 4G units. When I put them each on the Digi development board, using the same SIM for all of them, two work fine and two will not sync with the network. I have not been able to find any pattern regarding hardware revision, settings, etc, to match which ones work and which ones dont. Can you think of any reason this might be happening, or any troubleshooting steps to try?

Thank you.

Do they all have the same configuration, specifically the appropriate APN for that SIM (ATAN)? Are they all running the same firmware version (ATVR) and have the same modem firmware installed (ATMV, only available after the module has booted completely)?

Are you using the same style of antenna on all modules?

Thank you for your reply. Updating the modem firmware to 23.00.004 (M0A.00040) and correcting a problem with ATOA on one of them seems to have gotten everything sorted out.