@djbradster 

This behavior is actually a built-in feature of your Deco to prevent network conflicts. Here's what's happening: When your Deco's WAN interface receives an IP address from your ISP modem that falls within your configured LAN IP range, the Deco automatically changes its LAN IP address to the next available range to avoid conflicts. As a router, Deco cannot allow the WAN and LAN IP addresses to be in the same range, as this would cause NAT to fail. This automatic adjustment is designed to ensure uninterrupted Internet connectivity.

The good news is that you can prevent this by manually setting a custom LAN IP address that won't conflict with either of your ISPs. Here's how:

1. Open the Deco app and go to MORE > Advanced > LAN IP.
2. Set your desired LAN IP and subnet mask to a range that doesn't overlap with what either Frontier or Spectrum provides (for example, you might choose 192.168.10.1/24 or 10.0.0.1/24).
3. Save the settings. The DHCP server IP pool will be automatically updated to match the new LAN IP range.

For detailed instructions with screenshots, you can refer to: https://www.tp-link.com/support/faq/2331/

By choosing a LAN IP range that doesn't conflict with either ISP's WAN IP assignments, your internal network will remain stable regardless of which modem you're using.