They’re not US only, but WiFi channels 12 and 13 are not available for use in the US, so kit intended to be sold in the US, and that includes some imp001 cards, must not access those channels. We have imp001s that are set for ‘rest of world’ which can access those channels, but I suspect the impExplorer kit ones are targeted at US users. All you have to do is set your WiFi router to use a specific channel, from 1 to 11. That usually means disabling modern routers’ automatic channel selection. I’m in the UK and I got bitten by this some years back.
In fact, checking the BlinkUp LED codes page reveals that long red, short red, short red is the signal for ‘searching for WiFi’, so I’d bet your issue is the WiFi channel.
The green light you mention indicates that BlinkUp was successful - ie. the data was correctly received by the impExplorer from the phone, but the impExplorer is then having difficulty connecting.