Is the termination after sending two warning letters an unfair dismissal?

Good afternoon,

I'm an employee with my company for 4 years with an unlimited contract.

I have just renewed my visa last month but yesterday I received a termination letter from my company stating that based on my performance and 2 warning letters on the 6th of October and 21st of November.

I received a warning letter on the 6th of October but I didn't sign and replied with justification and on the 21st of November I didn't receive any letter.

Please advise, should I sign the letter, and can this be considered an unfair dismissal?