try to find people who have the game to play. I can not remember how many people need to be in a game to start. It's been a while since I've played it . I got the game upon release and the online was pretty much dead then.
This is the way things generally go with multiplayer games and the trophies required from them. Tomb Raider, like the majority of multiplayer outings for the year, did not offer a genuinely compelling social experience. For the most part, I think gamers were in it for the trophies alone. I've had discussions with at least a couple of people or more who mentioned as much.
There are ways to remedy this. With the game almost being a year-old now, hopefully as mentioned you can dig up someone who still has their copy of Tomb Raider and is willing to join in an online session with you for the trophy needed. Another way to go about it would be to buy the PlayStation 4/Xbox One version of Tomb Raider: Definitive Edition later this month. Doing so would mean you'd have to reacquire all of those trophies again, but since there might actually be people wishing to flock to the game for a first or second time this will at least present another limited opportunity to find people naturally active in the servers no matter the game mode.
Then there's always the off chance that Tomb Raider could make its way to PlayStation Plus as a freebie. Whenever this happens system owners start flapping heir wings toward the game of the month. If you're willing to take the chance on that, then you could hang onto your copy and see if the lobbies will fill up once more.