I'm not a network guru by any means, but to me it sounds like you've pretty much narrowed the issue down to your business. The only modem setting that could stop your traffic is allowing computers to use the net by the MAC address which would take effect immediately, not after a week. So that's not the issue there, especially since they changed the modem. Supposedly you might have mentioned if they had copied the settings of the modem, or if they do it remotely.
Good thing is that you're using Ethernet in all places instead of WiFi. That alone eliminates many potential issues. Another good thing is that you've ran your computer in several places without issues which tells that it works flawlessly if the connection is good.
A couple of questions: Do you always plug into the same port in the business modem? Some modems can have one port as bridged while the rest are routed. The bridged port is at either end, usually the last one. It should not pose any problems, though.
Also, is there any issues with other computers or printers connected to the business modem?
Yet another question: Do you get the IP address automatically (Windows default setting) or have you set it manually? If manually, then that might pose a conflict if the IP address you've been using gets reserved by someone/-thing else before you plug your computer in. Actually that sounds like the most potential culprit in my limited thinking!
For what I know about business vs. private internet here, the business services may have a little faster customer service. Other than that, they're using the very same lines and modems. The main difference I've noticed is that the business department salespeople talk about prices excluding VAT while the private side includes it in the prices.
