Sportage 2000 Automatic Box Problems

Hello, I'm currently experiencing a problem with my Kia Sportage, automatic, 4x4, gasoline, 2000 model.
I noticed it had lost power when accelerating, so I decided to change the spark plugs, fuel filter, and fuel pump. I thought it wasn't anything serious since the difference was minimal.
But two days ago, I was driving along fine, then I braked gently, and when I accelerated again, the RPMs climbed high, almost to 5000 RPM, and the transmission wouldn't shift. When
I start the truck in Drive, it responds well to first gear, but if I drive, the RPMs climb and it won't shift into second. When I put it in reverse, everything is fine.
I went for a drive, put it in Drive, and the RPMs climb high and it won't shift, but if I press the HOLD button, it shifts into second normally. Of course, it won't shift into third.
On cold mornings, it starts fine, reverse works well, there are no strange noises from the transmission, and it shifts into drive and reverse smoothly. If I press the lever for second or low gear, it engages that too.
It only fails to shift when I'm driving in drive; the RPMs climb high unless I press the HOLD button while driving, and then it shifts into second gear without issue.
Before taking the truck to a mechanic, I need some advice.
Any help would be greatly appreciated.

Well, friend, if you say the truck only has problems when driving, it could be a clutch or brake failure. On the transmission, there are test points labeled: OD, 2ND, RL, RV, UD

I recommend you check your points:

UD: low clutch 1.2
2ND: second brake

Do it with a pressure gauge, I also recommend checking the oil level and if you changed it, verify that it is the appropriate one.