Excellent question. The bottom line is that edge cases are determined by the community. If enough people think it's off-topic to vote it closed, and not enough people then think it's on-topic to reopen it, that means it's off-topic.
But I would look at it like this: the site is centered on computers. If the question is about computers or computer software, those are on topic. If it's about connecting things to computers, or about common computer peripherals, it's on topic; that's all about the things that make up the equipment complement for doing the work on the computer.
The grey area starts when you introduce off-topic platforms. Phones are off-topic, but we stretch the on-topic boundary if the question is about interfacing it with a computer, because the computer is involved. So things like syncing content, or using the phone as part of the communications link in which the computer plays a role, are usually on-topic.
But even there, if the issue is central to the phone, like trying to get an Android app working in order to facilitate the process involving the computer, that's probably better solved on the Android site.
So the printer is "once-removed" from the site's central topic, but it's allowed because of its role in using the computer. The phone is off-topic unless the question is central to, or directly involves, the computer. If the question's focus is getting a phone to work with a printer, the computer isn't involved at all.
So in this case, if the problem that needs to be solved is in the phone, I'd say that's off-topic. If the problem is in the printer, there are two possible cases. If the issue is something that is unique to connecting with a phone, it would be off-topic. If the issue is something generic that also applies to connecting it with a computer, then maybe. So take that case.
If solving that problem requires doing things, i.e., diagnostics or settings, on the phone, then solving the problem is off-topic. If the problem can be replicated on the computer and solved using the computer, or done entirely within the printer, an argument could be made for it being on-topic.
But in that case, it would probably be better to leave the phone out of the question. If the phone is relevant and important to the solution, and the computer is not, I'd consider it off-topic.
I just reread your question and answer. It turns out that the solution was in the printer, but the problem you experienced was getting the phone to see the network printer. I'm no expert on the Android site, but it just seems like that problem would be on-topic there.
However, the actual underlying problem is probably on-topic here, just tough to ask in a clearly on-topic way unless you had discovered it while trying to connect an on-topic platform. I did some wordsmithing on your question to better focus it; make it less off-topic and set it up for your answer.