Welcome to Lara's world,
The series is under construction which means the timeline is a bit wonky.
I recommend reading Razed before you dive into Trials.
Razed will give Lara's character more background information for you to understand what is happening in Trials.
This DOES NOT mean you can't read and enjoy Trials without Razed. Trials will slowly bring Lara's past to light, but if you are anxious for the information up front, start with Razed.