Ah, modem/router combos! They can be tricky. If it’s handling both modem and router duties, you’ll need to ensure the port forwarding is set directly on that combo device.
If you’re sure the forwarding is correct there, some combo units still route traffic oddly despite the settings. In that case, a workaround is to enable a feature called UPnP (Universal Plug and Play) on the modem/router if it’s not already on—it might automate the port handling.
Let me know if you need steps for UPnP or if any progress is made! 
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