I'm looking an unlocked phone or tablet, but I've ran into a problem.

An unlocked phone or tablet is not tied to a specific carrier, so you have the flexibility to choose your own provider. To use it, you need to obtain a SIM card from a mobile carrier of your choice and insert it into the device. The SIM card will provide your phone or tablet with a phone number and access to cellular services.

You don't necessarily have to visit a specific manufacturer like Samsung or Apple for setup; you can typically activate your device and get assistance from your chosen mobile carrier, as they can help you with network configurations and any technical issues related to your unlocked device.

You can go to any carrier that supports that device. You can order a prepaid SIM as long as the device is supported. If your device has eSIM support, you can buy an eSIM and receive it instantly. Go to the carrier website and they will have a place to check if device is supported before you buy a SIM. For example, here is where you'd check if a device is supported by boost mobile: https://www.boostmobile.com/check-compatibility

Check that IMEI number or you could have issues with some networks not working in the USA https://imeicheck.com/imei-check Some devices are made for networks outside of the USA and thus will not work here well. Like LTE 5G will not work at all as the internal hardware will not support it, it's missing all together. BUYER BEWARE