How do I make my iPhone my primary device for Google/Gmail?

I've always had an iPhone thru Verizon. For one month I was in an area with zero Verizon service so I got a cheap AT&T prepaid (samsung) that I will never use again, mainly to hotspot my iPhone off of. I'm back and I started noticing I am not getting my emails on my iPhone. The same gmail I've had on my iPhones for 15 years. I re-enter my password and it won't connect until I give a security code that was sent to my AT&T prepaid phone. How did that phone I used for less than a month take control of my account that I've used on my iPhone for 15 years? I want to completely remove any memory of that phone so when this happens again and I no longer have that phone or phone number I won't be locked out forever. My iPhone should be the phone that gets security codes sent to.