The fastest way in would be a CS degree. CIS is pretty generalised, so you may still have to prove that the CIS was sufficiently rounded to teach you all the core concepts required in your role. This discrepancy could be the difference between making a candidate shortlist, and not.
Once you get above 6-8 years experience in an industry, people will guague you based off the Experience listed in your resume. Until that point, they will be looking at your qualifications and looking for any other examples of your work, to try and figure out what you know and how good you are at it. A CS degree provides a solid baseline for a senior engineer to teach you more specific stuff, without teaching you the basics. When a qualification isn’t available, they’ll look to any examples you can provide - A StackOverflow or GitHub account is great for showing what you understand and what you can do.
If you’re largely IT based, I’d suggest some self-learning, but ideally you could do an open course out-of-hours to teach you deeper concepts and approaches to problems. If that gives you a qualification of some kind at the end which you can show to a prospective employer, all the better.