Have you tried it at your other place yet? Could you check something? After you install the OBS NDI plugin and reboot, can you run a speed test and post your results?
I'm running a dual PC setup on a gigabyte network. The ISP is AT&T, advertised 1Gbps up/down. In reality, I'm getting about 800-900Mbps up/down. However, after installed OBS NDI, my speed drops to <100Mbps, even without using OBS. I already posted on Palakis' GitHub page regarding the issue, as well as submitted a support ticket with Newtek. I just want to see if anyone else is experiencing the same issue.
TL;DR: I think you are saturating that switch, especially if it is an integrated unit that routes/switches or routes/modems/switches. You should get another gigabit switch for your NDI systems only, and uplink that to your main network or switch/router. Creating a separate segment will alleviate the overhead and switching of the NDI which in my case can go up to 300Mbps+ (Each!) between the two clients.
Are you using the integrated switch that is on the Router that AT&T provided? It's possible that you are saturating that switch with more traffic than it can handle in a timely manner. In my testing between two systems running at 1080P/60 Full 709 Color output, I nominally stream at 190Mbps and peak to 320Mbps just between those two boxes. If you take that into consideration when talking about from a switching perspective, you're sending up at let's say 200Mbps, while another is receiving down 200Mbps. Then at the same time if you're streaming out it's whatever that bandwidth is plus routing overhead on the CPU etc. Anyway, my point is that maybe you should segment the network and see what happens.
What I did was move the two systems using NDI onto it's own Gigabit switch together, so I didn't saturate the segment with all my other devices and such that are on the routers' switch ports natively, I did this as a precaution just in case the NDI API utilized a lot of broadcasting and such, this way if it did I could limit the broadcasts to that segment with the NDI systems. Then I uplinked that dedicated NDI System Group (Two PC's with OBS-NDI) Gigabit Switch to one of the gigabit router ports. In this way I'm not using the switching of the router to facilitate the transfers of the NDI traffic, and those systems when broadcasting can easily send traffic to the router through the uplink, which is about 15Mbps from one system and 5Mbps from the other, the first going to restream.io, the second to twitch.tv.
In my opinion most consumer products especially integrated routers that feature switch ports, and even more so integrated router/modems with switching, they just don't have enough juice to sustain high Mbps traffic while efficiently routing at a high Mbps simultaneously. In your case, if you're two systems (or more) are connected to the aforementioned integrated router/switch or router/modem/switch, I believe you are just oversaturating it above it's processing limit, which is resulting in lower throughput on the main WAN interface. Any off the shelf decent gigabit switch should solve your problem, I use a simple TP-LINK TL-SG108E on my NDI segment, that then uplinks to my ASUS-RT-AC56R Router which is then uplinked on it's WAN interface to a Netgear CM-700 Cable Modem. I'm subscribed for 300MbpsDL/35MbpsUL, and I can still pull that or more while streaming with NDI active and running both streams out.
I believe if you do this, the issue of limited bandwidth to the WAN/Internet should be resolved. Unfortunately it does mean making a minor purchase ($35-$50USD), but well worth it!