Via Vox:

Like Manafort, Patten worked for Ukraine’s pro-Russian political faction. Like Manafort, his payments went through offshore accounts in Cyprus.

Also like Manafort, Patten worked closely with Konstantin Kilimnik, a Russian national who Mueller claims is tied to Russian intelligence services. (Mueller indicted Kilimnik alongside Manafort this year for attempted witness tampering, but he is overseas and has not been arrested.)

Patten told the Daily Beast that he worked with Cambridge Analytica during the 2014 US elections including "hon[ing] Cambridge Analytica’s microtargeting operation " and on “several overseas campaigns.”

Mueller has long believed that Kilimnik, “has ties to Russian intelligence service and had such ties in 2016.”

According to the Plea Agreement, Patten was first interviewed by Mueller's team in May, 2018.
Prosecutions for FARA violations are rare -- there isn't even a sentencing guideline that addresses FARA directly.

Patten's plea agreement confirms he is cooperating in hope of receiving a benefit at sentencing. His cooperation will include further debriefings by government agents and prosecutors working with Mueller, including providing testimony to the grand jury or at trials.

One interesting item in his plea agreement: He's not allowed to make money for speeches, books, articles, etc about his illegal conduct.