This might be a stupid question,

[tonerhead]

Here is one thing to try while connected with TeamViewer:

1. Change subnet mask to 255.255.0.0
2. If you can then access the web page of a printer then either change its IP address or its subnet mask.
3. Or if you can login to the router, change it LAN settings to the old subnet.
4. Have your sister reboot her computer

Sorry, I don't think this would work as the subnet of the printers is still likely 255.255.255.0. The only options I see are either to change ip scheme of router or have the ISP do it. Or option 2 is to have the printers reset to dhcp. Or to walk your sister through setting the printers to the current ip scheme from the machine control panel. Personally I would contact ISP and have them do it. It was their mistake, they should fix it. Nothing pisses me off more than when an "IT" company comes in and replaces a router with one that has a different IP scheme and all of a sudden 40-50 people can't print/scan. In those cases I tell the customer to have IT contact me because if I need to make hours of changes, it will be billable.

[skynet]

If there is nothing of any importance on one of the printer's, factory reset, it will probably default to dhcp.

[GeneOR95]

Just a thought. I have Chrome remote desktop installed on both of my laptops. I can access either one from the other regardless of where I am in relation to the other. I can also use it from my Android phone and tablet to access either laptop. It might be possible to walk her through installing it on her computer and then you could access her computer directly and make the settings that are necessary. Worth a try.