I'll offer some observations on the examples in the question.
Two ISP's on one network - Help needed?
I'm not a networks guy so not qualified to comment on the question being too broad (the close reason). Questions can be a bad fit for the site for multiple reasons, so closure for a different reason than you might have picked doesn't mean the guidelines are confusing; it just means that more people picked up on a different one of the potential reasons.
The issue with corporate IT isn't that it's in a corporate setting. It refers to matters that are typically under the control of IT administrators and we don't want to interfere with their job or circumvent policies that may be in place for a reason. Matters in an enterprise setting that don't entail those kinds of considerations are generally on-topic unless they're really better suited to Server Fault. I had this very question early in my SU experience (What is the definition of "corporate IT support"?). Like much of the detailed guidance, this got explained in a Meta thread, but it would be beneficial to better explain it in the Help Center guidance.
Is it possible to force a device to sign onto the 2.4g network band with band steering turned on?
The OP clarified the question with an edit, "My question is about whether or not anyone has heard of manually directing a device to a single band if band-steering doesn't work well with it (but works for other devices) without turning band-steering off." The question attracted several close votes, but reads like a general question on networking. Again, I'm not qualified to judge this one, but to me, it reads like a typical networking question that's routinely considered on-topic, or at least close enough to commonly get a pass from the community.
I'm not sure it's possible to define things so cleanly that any and every question is unequivocally either on- or off-topic. The SE network relies on the discretion of trusted users, in a consensus, to decide things that fall in the gray area. What is on-topic and why is, to some extent, philosophy and culture, and it's that way on purpose to allow sites to evolve.
Comparing two broadband options (LTE with wireless router and DSL with a wired router)
Your answer was well received and the question was closed anyway. This is another networking question and I'm not qualified to judge it. Maybe one of the close voters can elaborate on the thinking behind their vote. But I would ask what kind of clarification in the guidance would help for a question like this? Some questions rely on subject matter experts judging how answerable a question is within the site's constraints.
actual repeater vs router-as-repeater
This one doesn't appear to be in the gray area, at least after editing. There are no close votes and everyone seems to agree that it is on-topic. It looks like this example was included because it seems to have similarities to #3, reinforcing the question of why #3 was considered opinion-based. I'll have to leave it to close voters on #3 to compare them.
How to compare two network adapters?
Looking at the history, only one or two readers thought this was a hardware recommendation question. If I read the timeline correctly, there were no actual close votes (or perhaps one or two that timed out). This isn't a hardware recommendation question and doesn't fall in the gray area. A purchase decision was the impetus for the question, but it doesn't actually ask which product to buy. It describes unexpected poor performance and asks how to compare adapters in a way that would reliably reveal expected performance.
This is the kind of detailed guidance that requires a Meta search to find, it isn't in the help section. There are canonical discussions, like Jeff Atwood's post Q&A is Hard, Let’s Go Shopping!, which describes asking for technical information that can enable you to make your own, informed purchase decision. There are a number of such discussion in Meta that are pretty fundamental explanations for what the help section guidance means. It would be helpful to users to summarize these clarifications in the Help Center.
How can one use CDMA(code division multiple access) in individual communications
This isn't in the gray area, you're spot on -- it's off-topic, and you laid out the reasons well in the question here. It doesn't need clarification in the guidance. This is another case where a small number of users interpreted the question as on-topic. That why actions like closure require a consensus of many experienced users (a moderator = many), and the system worked on this question.
Summary:
The site relies on discussion in Meta for clarification and detail. The help section sort of points people there for clarification, but new users rarely look at the Help Section let alone go searching Meta. Even many experienced users don't spend time researching what's available in Meta. Just a guess, users who get involved with community moderation, like working the review queues, are probably the majority of users who research these issues on Meta unless they are personally affected by an issue. To make matters worse, much of the Meta clarification isn't even in our own SU Meta, it's in SE Meta.
There's a ton of clarification out there (although there are still some questions that aren't definitively resolved even there). However, there is a relatively small number of fundamental clarifications to common issues that account for a substantial portion of the recurring misconceptions and unfamiliarity. Summarizing those within the Help Center would go a long way to clarifying the apparent gray area.