Worst Customer Service Experience - Bar None. How do I file a formal complaint?

A couple years ago, I attempted to activate a new gateway.  It never activated, and I spent a couple hours with a support agent, and eventually had them reactivate my old gateway.

I recently got another new gateway that I picked up from a local xfinity store.  I activated it this last weekend, and it went shockingly smooth.
I took the old gateway back to the store, and turned it in.
I quickly realized I didn't get any notices from my security camera (highly unusual) and when I checked they were all off-line.
I attempted to reset my modem through the app, and it showed off line as well.

I called customer support, and talked to one agent, who seemed to have an understanding, then got abruptly transferred mid sentence to a hard-to-hear agent with a lot of background noise.  I got that he was to help with log-in issues.  I told him that was not my problem... he transferred me to an "internet" agent, who eventually stopped responding to me altogether.

I stopped at the store, and with screen shots of my camera events, and the timestamp on my receipt pointed out that they apparently disabled my new gateway when they transacted my old gateway that was working just fine for nearly a full day.

The seemed less than concerned, and only said I'd have to go home and reactivate.

so I did...  and it would not recognize the 2d scan code.  I contacted support, and eventually got on a call.  The agent could not find my gateway in their system, said the old "black" gateway is what still showed on my account, and eventually told me I had to go back to the store (nearly back to my workplace, and during rush hour!!!

I talked to the same two store employees who again showed a shocking lack of concern, but I did come away with a new gateway.

when I got home again, I couldn't activate it as it said I needed "xfinity internet to continue".  
Flabbergasted, I attempted yet again to contact support, and requested voice support again.  Eventually I got it activated after another hour.

Needless to say, I had to set the [Edited: "Language"] thing up from zero (previously the activation retained my wifi name and password).

What an unmitigated disaster, and absolutely atrocious customer service.

I expect credit for the nearly 5 hours I spent (and lost productivity at my job).  
I at least expect a legitimate way to file a complaint for this fiasco.

I can guarantee I'll be looking for another provider.  and I will NEVER recommend xfinity.