Sorry for the trouble,
After checking the technical info for your doors, it seems they’re not compatible, as follows.
Chamberlain B750 is Security+ 2.0 system (that usually with yellow learn buttons when used on garage door openers).
Liftmaster ELITE series logic 5.0 is also a Security+ 2.0 system (see manual https://www.controlledproducts.com/customer/docs/skudocs/J501L5_Installation%20Manual.pdf)
So I’m afraid you will need to return 390 remotes, and check yellow learn button remotes instead.
Thank you.