Hi
Checks 2 things please (user manual here: https://www.downloads.netgear.com/files/GDC/MR1100/MR1100_UM_EN.pdf)
# ethernet data offloading should be disabled (page 33)
# usb tethering should be disabled (page 40) —> usb should be set to charge only.
If the data offloading is enabled (especially via the ethernet connection), then the ethernet port on the MR1100 effectively works like a LAN port.
It is still strange that the Airport get an IP address if a switch is connected in between.
Cheers!
Bogdan