Hang on, if you can't tighten OR loosen the truss rod there might still be hope. The complaints I've seen from other EHB owners was the the truss rod was bottomed out from the factory. If you can't move yours in either direction it might just be stuck. Truss rods sometimes get a bit stuck if they haven't been used in a while. Sometimes they need a little jolt to get them moving. Try using slightly more force, and give it a little jolt, but LOOSEN it when you do this. And then when you've (maybe, hopefully) managed to loosen it, you can start tightening it. If this works, go slowly. Do 1/8 of a turn, test/evaluate. If you still need less neck relief, tighten it another 1/8 turn, test/evaluate, etc. etc. Make sure to check the tuning every time you've tightened it. If you end up needing to tighten it more than 1/2 a turn (from where you originally started) it might be a good idea to leave it overnight to stabilize before you tighten it any further. A little bit of WD40 can help with loosening it, but a little goes a long way, don't make a mess!
Make 100% sure you use the correct size allen wrench. I believe Ibanez use 4 mm on all their basses, and they include a multi tool with a 4 mm allen wrench with all their instruments except their cheapest entry level models. Do NOT use the closest sized SAE (inches) allen wrench, which may seem like a good fit, but is in fact slightly smaller than 4mm, which means in can slip more easily and damage the truss rod nut.
Btw, a couple of months into the winter is EXACTLY the time when truss rods often need to be tightened a bit, to prevent excessive neck relief (and consequently higher action). This is perfectly normal and in itself no a cause for concern. It's not about the temperature, but humidity. Electrical heating inside your home will dry out the air even further, and make conditions even worse for musical instruments made of wood.
I noticed that you don't actually mention anything about neck relief, only that the string action is higher. High action can be caused by other things as well. The string action shouldn't even be considered until the neck relief is adjusted correctly. Action is adjusted by filing the nut slots and raising/lowering the saddles, but only AFTER the neck relief/truss rod has been adjusted. Higher action is a side effect of too much neck relief of course, but it's not the way your should evaluate neck relief. You should either measure it with appropriate tools, eyeball it by sighting down the neck (which may require some experience) or the BY FAR best method (IMO): Play the bass and gradually tighten the truss rod for the straightest possible neck that doesn't cause buzzing or choked notes in lower parts of the neck with YOUR playing style and YOUR choice of strings. You can read setup guides online which will tell you how to do all kinds of measurements and set up basses "by numbers" but the correct amount of neck relief will ALWAYS be the straightest neck you can get away with.
My method for my own basses is to simply tighten the truss rod until I start to get buzzing in low frets, and then SLIGHTLY loosen it. This gives me a PERFECT result every single time, and is a LOT faster than messing around with feeler gauges for a "setup by numbers". No setup guide can predict the correct amount of relief for any given neck, or the correct neck relief for YOUR playing style and YOUR choice of strings. Some necks/players/strings need more neck relief than others. The same goes for nut slot heights and saddle heights. There really is no "one size fits all" when it comes to bass setups. At least not if you want the perfect setup for YOUR bass/strings/playing style.
Then again, maybe really your bass really is faulty. This is impossble for me (or anyone else) to say without actually testing it. Unfortunately there does seem to be far too many complaints about the truss rod on these EHBs.
