A quick google search says, yes, there are Marines in Afghanistan.
Whether or not they have good internet depends on where they are.
Now the question is, why are you asking?
Did you meet someone online that is claiming that they can't call you or video chat? 99.99% chance that you are being scammed. Any idiot can make a profile on social media using the picture of some random Marine they found online. No member of the US military needs you to send them an iPhone, pay for a trip home, fill out their leave form, or anything else like that.
UPDATE: Based on what you say, I am now 99.99999% sure this is a scam.