Two boxes do not automatically mean a problem. Double NAT can occur when two routers both translate traffic along the same path. Google notes that many people notice no performance effect, although some applications and communication between separate home networks can be affected.
Draw the path before changing it
Write down the model of the provider's box and your own router. Draw which cable connects them and which device each phone, computer and printer joins. A second box might be a modem or an access point rather than another active router.
Consider a simple example: the laptop joins the newer router, while the printer still uses the provider's Wi-Fi. “Both have internet” does not establish that they share the same local network. That is a more useful observation to take to support than “the Wi-Fi is slow.”
Decide whether there is a symptom to solve
- Does ordinary browsing work normally?
- Is only communication with a local device failing?
- Did the problem start when the second router was added?
- What operating mode does each device report?
Do not switch modes from a generic checklist. A provider's equipment may serve other household services, and a mesh system may lose features in a different mode. Ask the provider or manufacturer about your exact arrangement. Keep the working configuration recorded before making any agreed change.