I've been having the same problem with mine, so I looked into it. I don't know where you live, but in my city, there are "hot spots" where 4G is available, and where only 3G is available. Oddly enough, the 3G seems faster, but won't download apps and stuff as you mentioned. I have to locate WiFi, as well. The 4G downloads data just fine, but since I live in Billings, Montana, the only 4G hot spots are few and far between.
The antenna in the MAXX might just suck, to be honest. I think it has dual antennae, which theoretically should improve the 3G-4G network performance, but who knows...wouldn't be the first time Motorola's come out with a crappy antenna receiver. Just know for now I suppose that you're not alone in your dilemma. A poor antenna signal is probably what's causing it, and really, if it's the manufacturer's fault, there's not much you can do. I'm in the same boat with ya...I just use WiFi and grind my teeth, wondering why I paid $700.00 for it. Then I remember I love my Droid MAXX, despite [his] flaws (haha).