Just had to replace mine due to cold temps and the plunger sticking. What worked for me was allowing it to heat up. I parked it in the sun at work, with car unlocked I pushed the door a few times.. came out after work and the actuator eventually had released and popped the door open.

For anyone reading this about the red pull cord in the trunk, that's an electrical failsafe. It's mechanically attached to the actuator to release the lock, it won't help a frozen plunger.

From what I can tell mine was from the factory, date stamp on the part 2017 .. I guess 8 years isn't too bad. Part number I took off the car was 4L0862153D, FCP and ECS couldn't confirm 8V0862153B fits 2018 S6 but it's listed as a superseded part on audiusa.com:
https://parts.audiusa.com/p/70168723/8V0862153B.html