The easiest way to sniff out ISP meddling is to call 'em up and ask directly if inbound traffic on port 25565 (TCP and UDP) is blocked on your connection. Some ISPs impose restrictions unless you ask real nice—or upgrade to a “business plan.” 
If you’re feeling techy, you could also run a simple port test. Use an online tool like “CanYouSeeMe.org” and punch in your public IP and port 25565. If it says the port is closed despite your heroic port-forwarding efforts, it’s likely your ISP playing gatekeeper. Give that a whirl and let me know! 
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