Cannot send images over wifi

Exact same thing is happening to me. I have the same Arris TP1602 and Netgear R7000P. I'm pulling my hair trying to figure this thing out. I went so far as to plug the Arris directly into my laptop and turn the laptop into a mobile hotspot for the iPhone (bypassing all Netgear equipment). Same thing - rediculously slow picture send speeds on iMessage (2-4 minutes per image). So I can say with near certainty that the issue lies either with the Arris TP1602 or Optimum Online. I have the 400/40 speed plan, and this happened to me about a week or so ago. And only (seemingly) via iMessage and pictures (don't do any iCloud stuff). I put back the original configuration and pulled up the graphs from my Cisco firewall and it seems very clear to me that there is a throttle for iMessages at about 250kbps with our hardware configs. I can upload to Slack or Google Hangouts the same image and I'm getting ~30Mbps upload speeds via wifi on the iPhone, but pictures to iMessage are capped at about 250Kbps.

I know this doesn't so much help the current situation as maybe eliminate some variables for you. Take solace in knowing you are not the only one with this problem, and if I come up with a solution, I'll let you know (and 2 hours on with Optimum support today wasn't the answer - shocker).