Tesla is offering retrofits for $200 via "accessory install" appointments. Put "LTE retrofit" in the description.
3G Cellular Network Retirement | Tesla Support
AT&T has recently announced that it will discontinue its 3G cellular service by February 2022. Once AT&T’s 3G service is discontinued, Model S vehicles built before June 2015 without a 4G/LTE capable modem will not be able to enjoy some features unless connected to Wi-Fi.
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