As long as you configure channels you are allowed to use in the US it shouldn't be a problem. Otherwise you might have issues with some clients not being able to connect to your wifi.
Depends where it comes from. The vendor should have a data sheet saying what frequencies are available in which regions. You will then have to disable the frequencies not permitted in your country. Or flash a local firmware onto the device.
You might run into trouble with plane radar systems. Here we have built-in features that disables certain frequencies when the router notices such activity.
In the worst case, you will get a visit by the FCC and they confiscate your device. And they will send you an invoice for their trouble.
Most routers have a WiFi region setting to deal with that sort of stuff, but if it doesn't look up what channels are allowed in the US and manually set it to use those.