That's not accurate. You don't need to upgrade the MCU/infotainment computer and the retrofit is only $200.

"A source has informed me that due to AT&T shutting down its 3G network by Feb 2022, Model S's without an LTE-capable modem will lose connectivity features unless connected to WiFi. HOWEVER, Tesla will offer retrofits for $200. Affected vehicles are pre June 2015."

"Tesla will be offering retrofits for these vehicles for a bargain price of only $200, even though the vehicles affected by this could be up to nine years old."

Source:https://twitter.com/SawyerMerritt/status/1415851250055323648