Hello, jose
Tibia has an anti-afk tool that kicks you out after 16 min without you taking any actions, no matter what (an action is anything that obly you to click on your mouse/keyboard.
But if you intend to make a single action every 15 minutes, there are some ways to train being semi-afk (note that there isn't a perfect way and any player can be a jerk and kill you or the monster you are attacking)
Being premium acc you can do the quest shadows of yalahar and get access to the monster called modified gnarhound. While you can't train your shield (it does not attack) it has high HP and high resistances, so an enchanted melee weapon (such as fire sword, fire are, etc) will be needed.
If you're free, you can use the mad sheep method (recommend to go to hidden places so you won't be bothered), but you need to be low level (max level 14). You can do this with any skill, low to high, but your level must be less than 15, otherwise you'll outdamage the mad sheep's healing skills.
An alternative method if you got a lot of money is to use the training weapons, but you won't up your shield.
Hope I helped. If you got any questions, just do and I'll try to improve the answer