There is actually an example on their website.

ntfy ssh login alert

In addition to what others say, I also have ntfy notifications on successful login.

You’ve got to love the open-source community! If only the whole world worked like that.

Any good alternatives?

I also had the same problem with Chrono and some other alarm applications. I think the ones that worked well had SCHEDULE_EXACT_ALARM in their permissions, but of course, that’s just a guess. This one is my favorite: https://f-droid.org/packages/com.best.deskclock/

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Maybe I should first ask the cellular provider for IPv6, because I misspoke; they support it but haven’t enabled it for me. If they provide it, then I won’t currently have a need for IPv4, but thank you for the advice.

The problem was with DS-Lite tunneling, as some users mentioned, and it only works over IPv6. However, now I have another issue. My entire family has access through their ISPs, but my cellular data ISP does not support IPv6. Is there any workaround that doesn’t require me to look for a new ISP or asking for IPv4 address? 😀 By the way, thanks to everyone for the help!