Change or Disable Private IP of a Lightsail Instance

You cannot disable the private IP address of a Lightsail instance as it's a fundamental part of how AWS networking works. Each Lightsail instance automatically gets a private IPv4 address that is used for internal AWS communications.

However, there are several ways to address your issue with T-Mobile users being unable to connect:

1. Use the public IP address instead of the private IP for external connections. The public IP is what external users should be connecting to, not your private IP. Private IPs (like your 172.26.x.x) are only meant for internal AWS communications and shouldn't be directly accessible from the internet.

2. Attach a static IP address to your instance. This gives you a fixed public IPv4 address that doesn't change even if the instance is stopped and started. This is the recommended approach for hosting services that need a consistent address.

3. Enable IPv6 for your instance if it's not already enabled. This provides an additional addressing option that won't conflict with the IPv4 private address space.

4. Check your firewall settings to ensure they're properly configured to allow connections from all required IP ranges.

The issue you're experiencing is likely not related to your instance's private IP address but rather how your application or service is configured to handle external connections. Your private IP (172.26.x.x) should never be directly exposed to external users, including T-Mobile customers. All external traffic should be routed through your public IP address.
Sources
Networking - Amazon Lightsail
View and manage IP addresses for Lightsail resources - Amazon Lightsail
Connecting 2 Lightsail on Same regions via Private IP. | AWS re:Post