This is as good as I've been able to do....
I was about ready to throw my phone in front of a fast moving freight train after a couple of months of trying to locate proper h2o APN settings. I tried the customer service approach many times, as well. I searched the endless expanse of the web, only to find disappointment at every turn. The only reason i wanted h2o was because their web site advertised 4G coverage for my area, and even though they throttle it back after 2.5 gigs, their unlimited data plan is still better than AT&T's. I brought this up to one of the many CS reps I spoke with. About halfway into a an hour long discussion the rep told me that 4G wasn't possible because they didn't have the technological capabilities to provide 4G coverage in my area. I told him that their coverage map indicated otherwise. He then told me that an address had to be typed into the search box to find the coverage for my particular area or the map would just show 4G coverage everywhere. For a moment, I actually believed that I might have the ability to reach into the phone and across the airwaves to strangle him, but I remembered that he's just some poor schlub being paid a piss poor wage to read prompts off a computer screen. In the end I just started playing around and using different combinations of all the configurations I found. Here's what worked best for me in my home area(central Texas)on 2 Samsung Galaxy S4 i337 v4.3 and v4.4.2, Samsung Captivate i897 v2.3.5, Samsung Reverb m950 v4.1.2 and ZTE n860 v2.3.5.
Name: H2O Phone
Proxy: Not set
Port: Not set
Username: Not set
Password: Not set
Server: Not set
MMSC: http(semi-colon)(forward slash x2)mmsc(dot)mobile(dot)att(dot)net (Sorry, I can't post links yet, But I'm sure you know what to do...)
Multimedia Message Proxy: 184.108.40.206
Multimedia Message Port: 80
Authentication type: default,mms,supl,hipri *(No spaces after the comma's)*
APN protocol: IPv4
APN roaming protocol: IPv4
MVNO network operator type: None