Please Help!

I have a 2015 Ford Fiesta SE. It has the smart shift transmission.
The transmission started slipping between gears. I replaced the clutch actuator with ones I bought from Amazon and it fixed the problem
... for about 4 days. My car spun while going up my driveway amd the slipping started again. My husband seem to think if I buy the FORD clutch actuator, that will solve the problem. I'm not convinced. And I don't want to spend $500 on parts and labor if it's just gonna happen again because it's something inside the sealed transmission.
I got a letter from Ford saying my transmission was under extended warranty due to issues with the transmission but of course when I took it to Ford they said it wasn't anything covered by warranty. They wouldn't tell me the codes it showed and said my engine light was coming on because of my airbag.
I had a new serpentine belt put on about a month before the issue started. Now my AC won't get cold at idle... only when being drove. I don't know that this has anything to do with it but thought it might be useful to know

2015?

And they won't let Ford (their sponsor in all of this) honor? Might want to take receipts and get a hold of Ford directly and show them some of the work this dealership is doing. For if they are a Franchise versus a Corporate storefront - Ford would help you find a dealership willing to help.

The "slipping" can be caused by many different issues accumulating into one noticeable problem of being it has problems with shifting.

They may ask you for repair records and one thing they would ask nearly the first time - every time, is if the Battery has been tested - checked or replaced - and when?

When a actuator works, it needs a lot of current (Electrical power) to operate motors in these actuators to push the gears in and out and work a clutch system that mimics you as if you were driving a manual - only it's done by itself using a computer - called Transmission Control Module or in short TCM. This unit does the work of making the transmission make the car go.

When you have a bad battery, or the alternator/charging system needs work, the power it can deliver to the TCM to make it work is reduced and can hamper it's ability to stay in gear let alone shift into another one. So they will want to run diagnostics on the car if Ford is willing to help send you to a dealership they can work with, to get your car straightened out.

Could a serpentine belt being replaced cause the problem? If it wasn't put on right or was too tight. A mobile mechanic came out to put a new belt on... everything was fine. I watched the cameras and saw him putting it on my car with a crowbar(this was after the repair and him leaving). I text him and asked him why he was using a crowbar and he said it didnt have a tensioner. The belt has no give at all to it. It started making a really weird sound after we put the new actuators on it. 4 days later... slipping again.
It is a 2015.. I seriously think it falls under the extended coverage but the service department at our local Ford is a joke. They wouldn't tell me specifically what was wrong with it but it would be $2500-$3,000 for parts and labor.
I definitely appreciate your response. I will get ahold of Ford tomorrow and see if they can be of any help!

They would have had to remove the belt, there are several methods - up to and including the Alternator being removed to get the belt off their pulleys.

If they didn't put it back on right or damaged the alternators pulley trying to remove the belt - this can cause the belt to slip and even affect shifting - your headlights when on would dim when you put any sort of load on the car either by driving or turning the steering wheel. The car uses a Electric power assisted Steering - meaning; it doesn't use a power steering pump, it uses electrical power - from the alternator, to supply those motors the power they need to turn the wheels so you can drive.

So if it has problems getting power to make it go, your car can't supply enough electrical power to make it do what you want causing the shifting problem and loss of drive-ability condition