If you call or go in person, take notes about the day/time you spoke to someone, the name and job title of the person you spoke to, and what each of you said.
If you contact them in writing (email, online chat, etc.), note the dates you sent and received messages, the name and job title of the person you corresponded with, and what each of you said (screenshots work fine).
If you do not get a response from the bank after a few days, you can come back and move forward with the complaint anyway.