Instead of putting your trust in a certification – except as a perhaps-worthwhile avenue for self-study – I suggest that you apply for positions ... starting
within(!) your present(!!) employer ... which will allow you to branch out into new areas-of-interest. With your "telecommunication background," you probably know much more than you realize about "network security," and in any case you know about The Big Picture.™
Apply for positions, expressing your interests and emphasizing the business value of your
present experience, which is probably quite considerable.
Don't under-sell yourself. Don't focus on what you don't know how to do – instead, sell what you
do! Your (present or new) employer may well be pleased to train you (at their expense) on what you do not yet know.
Of course, certification study materials
are a valuable and fairly-concentrated source of information. I'm not "dissing" them on that point. However, I think that you should realize that "a Certification is not a Golden Ticket." What you would bring to your new position,
certificate or no, is most-of-all the business value of what your
present experience ...
and experiences ... have taught you over the years that you've been doing your present job.
(You know that "School of Hard Knocks?™" This is where it pays off ...)