How to gain weight while training triathlon?

I’m 33 years old, 5’11 height, 70kg in weight. I need some advice how to gain mass to I can build muscles to support swim, bike, and run.

Short story: Triathlon is one my bucket lists or should I say I wanna make it a lifestyle. I’m into the sport for about 7months now and from a cycling enthusiast for decades, I recently finished a 70.3 Ironman like a month ago. Managed to finished sub-7.

My body type is ectomorph and I’m having a hard time gaining weight. Not to mention my night shift schedule at the company I’m working at (work from home).