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Regarding the issue of yours, I would like you to double-check what type of IP address are you getting from the Sim card or from the WAN source you are using. You can get more information about IP addressing in this link: Private and Public IP Addresses - Teltonika Networks Wiki (teltonika-networks.com)
Next, I want you to double check if the interface for the PI4 is having a default gateway IP address?
Let me know these details :)