Cell Phone Plans That Work on Both Sides
Anyone who splits their life between San Diego and Tijuana runs into the same early problem: their phone works fine on one side and gets weird on the other. It is fixable, and the fix does not require carrying two phones.
Why your US phone gets flaky in Tijuana
The big three US carriers, AT&T, T-Mobile, and Verizon, all technically include Mexico in their standard plans now, usually as unlimited talk, text, and data with no separate roaming fee. In practice, coverage right at the border is inconsistent. Zona Centro and areas close to San Ysidro can see your phone bounce between a US tower and a Mexican partner network, and that handoff sometimes drops data or slows it to a crawl. AT&T roams onto Telcel or AT&T Mexico depending on where you are; T-Mobile and Verizon have their own partner arrangements. None of it is broken exactly, it is just less stable than being solidly on one network.
If you are just visiting for a day, this is usually fine to live with. If you are actually living or working from Tijuana regularly, it is worth doing better than "usually fine."
Getting a Mexican SIM
Telcel is the dominant carrier in Mexico and has the widest coverage in Baja California, which matters if you are moving around the city and not just staying in Zona Rio. A prepaid Telcel SIM, sold as an "Amigo" chip, is available at OXXO, other convenience stores, and Telcel's own shops. You will not usually see it displayed, ask the cashier directly for a "chip Telcel." Activation runs a small fee on top of the SIM cost, and you then load a plan, the Amigo Sin Limite package is the one most people want since it bundles data with unlimited calls and texts within Mexico.
The whole process (buying the SIM, activating it, and loading a plan) usually takes under 15 minutes and costs less than a single day of most US international roaming add-ons. AT&T Mexico and Movistar are the other options if Telcel's coverage does not suit you, but Telcel is the safe default almost everywhere in the city.
The dual-SIM setup
Most phones sold in the last several years support two SIMs, either two physical SIM slots or a physical SIM plus an eSIM. That means you do not have to choose between your US number and a Mexican one. Keep your US SIM in for calls, texts, and apps tied to your US number, and put the Telcel SIM in as your data line while you are in Mexico. Your phone will use whichever line you set as the data default, and you can switch that in a few taps.
This setup is what most cross-border commuters actually land on. It solves the two real problems at once: reliable, cheap data in Tijuana, and no interruption to the US number people already have for you.
What to actually do
If you cross occasionally, check whether your US carrier's Mexico coverage is already included before assuming you need anything extra. If you are in Tijuana often enough that spotty data actually bothers you, get a Telcel SIM, it is cheap and the setup takes minutes. And if you want both numbers live at once, confirm your phone supports dual SIM before you buy anything, since that one detail decides whether this is a five minute fix or a phone upgrade.