@Beatriz *********
“and you arrive in Thailand, you will need to register with the immigration office within 90 days”
Sorry, you got your wires crossed
Once you arrive in Thailand, you will need to get registered at your accommodation within 24 hours in the TM30 system.
If you stay in a hotel, they will do it for you. If you stay in a private property, the landlord or host will have to do the TM30
You DON’T “register” with the Immigration after 90 days.
All you need to do is making a “90-days reporting” on the day that is your 90th day after entering Thailand. You can report in person up from 15 days before or until 7 days after the report becomes due