From testing I have been doing if the connection can pass portquiz for port 3479 then Helldivers will work online even on PS3 and PS-Vita.
Using either my work AT&T Hotspot or my Verizon Smartphone as Hotspot and 3479 is not blocked so Helldivers works online. If I switch my connection to my AT&T U-Verse connection, I am not able to use portquiz.net to 3479 successfully (connection times out). In this scenario Helldivers will also not work.
I tested my PC & PS4 and verified that these also cannot get to 3479 using my AT&T U-Verse connection, but the PS4 / Steam version of Helldivers are able to play online without issue so must be using something other than 3479 to connect to Helldivers online.
Looking into port 3479 & AT&T, apparently this was AT&T’s chosen port for providing Remote Assistance. Basically a backdoor they would leave open but also something games like Call of Duty would use. Plenty of older posts can be found with people either complaining or inquiring about the issue.
Seems AT&T continue to use 3479 for their remote assistance, but now (more recently) changed things so they don’t leave it open and that is why Helldivers online stopped working for PS3 and PS-Vita.
If looking at registered ports on Wikipedia
I can see that PlayStation Network unofficially has 3479 and 3480 registered.
So I test 3480 from my AT&T U-Verse network and it is not blocked
Maybe best solution to this problem is for a patch for Helldivers on PS3 and PS-Vita to stop using 3479 and instead use 3480. Maybe it is some patch that PS3 and PS-Vita firmware need so they don’t try to use Playstation Network anything on 3479 even though they have unofficially registered its use.