Here is the process:
* Obtain an IMEI for a Mobley that is not currently activated on plan you own. The key here is when you activate it, AT&T will look for this IMEI being used on another plan. If they find it, they *could* flag it for investigation. I have heard of others getting multiple accounts with a single mobley, only to lose their older accounts due to violation of TOS. I do not have first hand knowledge here, but feel that paying $75-250 per account for the benefit is worth it.
* Mobley's can be obtained from ebay without risk.
* once you have the IMEI, you can then proceed with the next step of registering the plan. You do not need the physical device. Contact the connected car plan group, referencing the FCA discount (see other forums for this info, I don't have the link readily available). Setup the account as post paid, by providing your SSN / TaxId, etc. The discount for autopay should bring the entire bill to less than $25 with taxes.
*Once the plan is activated, you can choose to either have AT&T mail you the sim, or to notate your account that you can pickup a sim for no charge at the AT&T store. I prefer picking up the sim at the store. You walk in, reference your account number and the need for a sim. Explain this is for a connected car plan, so the device is in "your other car" and you just need the sim. They will give you a credit card size plastic sim card. The sim can be removed and has various sizes.
*DO NOT PLACE THIS SIM IN THE MOBLEY EVER - It will become locked to the mobley without the ability to unlock. There are various discussion on obtaining the PUC code, but I have gone down that route so many times and failed that I cannot recommend it.
*place the sim in any hotspot device you desire and enjoy fast, unlimited internet.