NOTE: Make sure your bot phone and your personal phone have free data roaming or these steps will end up being very costly.
The phone in the brain of the MarkSetBot is a Nexus 5X or Nexus 5. The phone is able to be set manually to a specific carrier.
As a first step, enable data roaming via Settings>Network>Mobile network
In order for the feature to work, your SIM needs to be enabled for "Manual Network Selection" or "Manual Carrier Selection". If you are using AT&T, you need to call support, ask for international support, and then ask for ATS (Advanced Technical Support) team. The change just takes them a few minutes
Once it is enabled, you will be able to go to Settings>Network>Mobile network>Advanced and have a section that lets you turn off automatic selection and pick a network.
The feature takes a fair bit of time to search (as much as 5 minutes). If the search fails, you may need to reset your network connections in the phone.