The ip address is dynamic for mobile broadband and it changes whenever you connect and disconnect the internet. However, there is a IMEI for every dongle. My quesiton is whether amazon would be able to obtain the IMEI to identify you.
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I don't believe websites can see the IMEI, which is a rather private "ESN"-like hardware ID for any device containing a cellular radio (IMEI is one of the successors to the old-school ESN). I think these devices have hardcoded MAC addresses that can't be changed. Other forum users have one dongle per account, so I would lean toward that if I were you.