I believe you are SOL for Pell grants my friend. The EFC (expected family contribution) caps your award, even outright disqualifies you if it exceeds $3,850. This is chump change mind you, which is why you didn't qualify for it the first time around when you got that fancy [proverbial or literal] aviation degree you wasted 4-5 years on. If your family is not on Medicaid you ain't getting a pell grant brother.
As to the question of already having a degree, only post-bachelor "teaching credential" programs are the kind of coursework that qualify for pell post BA/BS. So yes, you're out of luck there too.
The question for you then becomes, what is this network engineering certificate worth to you when it comes to the amount of skin you're willing to cough up for it?
Take me for instance. I have BS's and MS's in engineering, don't practice the field, currently chasing a permanent civil service job and in the process make squat for money. I'd love to go to dental school and be able to afford my own real airplane (not the barely legal flakin' paint buck fifty I currently drag through the sky..) and finally get the professional aviation 'need' monkey off my back. However that takes a ton of unsubsidized loans and a time commitment that I, at the age of 30, am unwilling to forego in light of my previous 8 year schooling (read wasted 20s..jon gosselin middle age crisis at 30 anybody?). So no dental school for me. That's called opportunity cost assessment. If the govt declared dentists evil tomorrow and forced the floodgates and allowed me to train for free, my opportunity costs would be dramatically different. The same applies to you. This is a good exercise for all of us as it illustrates how people actually do a sensible opportunity cost assessment of returns on investment when faced with a real element of scarcity. This is to say, when the interest free loans are no longer flowing, people actually put real weight behind their professional investment. Hell, not having parents that qualified for PLUS loans could have very likely help you avoid making the mistake of getting that useless degree the first time around. That's hindsight and I recognize that, but bear in mind your original question suggests you haven't learned anything from your first aborted takeoff that was your college degree.
Put simply, if the ROI for the training is NOT greater than the loan shark term personal loan you'd have to take in order to afford to get said training, then forego the training. Subsidizing worthless training (like your aviation degree) is not good for you or anybody else. Good luck.