After you beat the game you go to BB base and talk to the dog and all three of the BB bandits will want to challenged you to a fossil battle and when you beat them you get a note saying Frigisoarus was sent to VMM and thats how you get Frigisoarus
How To Get Frigisaurus In Fossil Fighters?
Defeat the 3 bb bandit bosses. They all have rank 12 vivosaurs so be ready
After you beat the game go to bb base and defeat the new bb team.
Beat the BB team again after beating the game to get Frigisaurus. Seriously, I just read the guide for my answer I AM NOT LYING
To get a t-rex you have to go to the parchment desert then get all 4 oasis seeds planted (by old dude) then search and you'll get lucky BUT I don't know where the rest of the four t-rex fossils are
Go to cold feet glacier