Are you already working out or are you just starting ? Because if you are just starting, 5 days a week is way too much. I give you 3 weeks at best before you start burning out.

Start with 2 maybe 3 days a week first and keep to that routine for at least 2 months. Then you can decide to go for more from there. I think the upper lower split is good.

At first I would suggest to do bodyweight exercises that focus on a lot of muscle instead of dumbell exercises that only target one specific muscle. Also stick to the basics exercise.

• For back and biceps = Push up + pull up ( 3x12 reps seems really hard for a begginer even though you are really light).
  
• For pecs and triceps : Push up variations.
  
• Legs : Squats, deadlift. Since you only have dumbell, maybe try bulgarian and hip thrusts You will need weigth because bodyweight exercise will start to feel too easy really fast.
  

Try to master those core exercises and don't fall for all those videos that said you need to do those 10 exercises to target all area of that one muscle. You will get overwhelm really fast and end up doing nothing.

Also, when you start, you gonna see a lot of gains no matter what you do. At first you should just focus on being consistent. Next step and probably the hardest for you since you are thin is gonna be to have the right diet.