With 5 on 2 they can dive you no matter what. You have to leave tower. Likely the enemies are fed as well.
In that situation it just sounds like you have to try to stall out the game. The question isn't what to do at this point, its what could've been done earlier.
I know thats not very useful, but thats unfortunately the answer.
Analyze what you could have done before the point of no return. For instance, maybe 4 man invading the enemies buff level 1 since you are braum. Forcing some flashes.
Ultimately you need your team to stop feeding once they've died say twice, so you can stall the game.
The best advice I have is to either play jungle, top, or mid instead. I know, thats sucks, I've had to change myself from being a support main. Its just not reliable in low elo. OR play carry supports, like brand or annie, and get good at it.
Don't worry about your adc, if hes good, he won't flame you for going carry. If hes bad, you're getting flamed no matter what more than likely. Just don't worry about it, mute him if you have to and do your best to carry. Any adc that is flaming his support is bad, I'm guilty of it myself, many times. I'm often right as well, but flaming someone is beyond retarded. It will only make them play worse and learn/focus less.
Just don't take cs, don't push lane unless you know what you're doing. However if the lane is pushing in uncontrolled help him control it, lol.