Time under tension is legit, therefore keep consistent with your training programme in the gym.
For adding weight, 'Caloric Surplus,' meaning eat more than your maintenance. Eat more nuts and dried fruit, don't be afraid of eating more nutrition which will help for the bulking process. Layne Norton has a PhD in Nutritional Sciences, one of the most respected and knowledgeable individuals in the fitness industry. I recommend going through this playlist on YouTube and meticulously take notes.
The key is consistency, be consistent with diet and nutrition and results will come.