Team changes won’t save
 

New to the game so apologies if I’m doing something obviously wrong. Every time I buy some player cards or open some packs, I activate the new cards and update my lineups. Then to lock these changes in I click Submit Team. When I leave the app and come back, all my changes are gone and my team has reverted back to how it was. My new cards are back to being Inactive. Any ideas?

you might be doing this when the game is simulating a day
 

If so, it brings the previous day's roster and setup back with the new league download.

I’m in Day 0 with two hours until a new league starts. Could I perhaps need to wait until the first game of the league has simulated?

You should be OK on day 0 before the first sim. But, do not make changes on one computer and then go to another computer or to your phone before the next sim. Other devices won't know what you did on the first device until the next updated sim is processed. And, if you do check on another device it will probably wipe out the changes made on the first device. This is because the default is to automatically submit when you logoff, so the 2nd device will wipe out the changes submitted by the first device.