Can't play Halo 5 online with 2 XB1s in the house

I fixed my lag from TWC by switching TWC’s modem/router into bridge mode and setting up a new router, but now neither I nor my roommate can play online if the other’s XB1 is on. It lags into unplayability or straight up lags out. I’m going to repost what I put up on reddit here.

I was one of the folks affected by the massive lag for TWC customers. I managed to get around that by putting the modem/router in bridge mode and hooking up a router I had in the house. Now I have a new problem.
My roommate and I can not play Halo 5 online at the same time. Or even one of us play Halo 5 while the other person has their Xbox One on, whether they are playing Halo 5 or not. The result is a game filled with long (upwards of 40 seconds) lag spikes and frequent disconnects. Halo 5 is the only game where this has been an issue. We have hundreds of hours playing the other games like Battlefield 4, Battlefield Hardline, Destiny, and a host of others. None of them have ever been an issue.
I know other people are playing from the same network just fine, so I know it’s possible. I just can’t figure out how. If you’re running two Xbox Ones from the same network and running Halo 5 just fine, I would be very grateful if you could tell me what router and/or modem you use, and possibly your isp and general location (I don’t need your address, but I see a lot of folks having this issue being in NY state like myself.).
If you would like to help me troubleshoot, my knowledge of networking is moderate. Most of what I know has been learned along the way trying to get games to work properly.
Here’s what I know so far:
1. It is not an issue of bandwidth. This seems to be one of the first suggestions, but it just can’t be. Not only have my roommate and I played online in several other games without an issue, but we’ve done so while our 3rd roommate has been online on his PS4 and his visiting father was streaming HD video. It isn’t bandwidth. It has something to do with the way Halo 5 handles network traffic, but…
2. It is not a NAT issue. Every device in the house has an open NAT type. I’m currently running a Netgear WNDR700 that I bought a few years ago when I was trying to get an open NAT on two Xbox 360s at the same time. I just ordered the Netgear N750 (WNDR4300) from Amazon, but it occurs to me that I don’t know for sure that will work.
3. It is something unique to Halo 5. Or possibly Halo 5 and Halo MCC. My roommate did not pick that one up, so I don’t know about that one. That doesn’t change the fact that I don’t know what the issue is that is keeping us from playing together.
4. uPnP is enabled. This could probably be listed under “Not a NAT issue,” but it gets suggested enough that I just wanted to get it out there.
tl;dr I need help getting Halo 5 working with two Xbox Ones on the same network when everything else seems to work fine. If you have any suggestions that I didn’t think of or could do me a favor and tell me what equipment you’re using to accomplish this, I’d be very grateful.

Does it still do this?