iphone 15 pro never connected to 5.1GHZ wifi, it only works on 2.1 GHZ offering appalling speeds

Hello kas_0070,

Thank you for reaching out in Apple Support Communities. You may need to modify the settings for your router that can help. Here's how:

Network name (SSID)

 Set to a single, unique name (case-sensitive) for all bands

The Wi-Fi network name, or SSID (service set identifier), is the name that your network uses to advertise its presence to other devices. It's also the name that nearby users see on their device's list of available networks.

* Make sure that all routers on your network use the same name for every band they support. If you give your 2.4GHz, 5GHz, or 6GHz bands different names, devices might not connect reliably to your network, to all routers on your network, or to all available bands of your routers. If your router is providing a Wi-Fi 6E network that isn't using the same name for all bands, Apple devices that support Wi-Fi 6E will identify the network as having limited compatibility.

* Use a name that's unique to your network. Don't use common names or default names such as linksys, netgear, dlink, wireless, or 2wire. Otherwise, devices that join your network are more likely to encounter other networks that have the same name, and then automatically try to connect to them.

Radio mode

 Set to All (preferred), or Wi-Fi 2 through Wi-Fi 6 or later

Radio mode settings, available separately for 2.4GHz, 5GHz, and 6GHz bands, control which versions of the Wi-Fi standard the router uses for wireless communication. Newer versions offer better performance and support more devices concurrently.

It's usually best to enable every mode offered by your router, rather then a subset of those modes. All devices, including older devices, can then connect using the fastest radio mode they support. This also helps reduce interference from nearby legacy networks and devices.

Channel width

 Set to 20MHz for the 2.4GHz band

      Set to Auto or all widths for 5GHz and 6GHz bands

Channel width specifies how large of a “pipe” is available to transfer data. Wider channels are faster but more susceptible to interference and more likely to interfere with other devices.

* 20MHz for the 2.4GHz band helps to avoid performance and reliability issues, especially near other Wi-Fi networks and 2.4GHz devices, including Bluetooth devices.

* Auto or all channel widths for 5GHz and 6GHz bands ensures the best performance and compatibility with all devices. Wireless interference is less of a concern in these bands.

This may help you in this scenario and is outlined in this article if you need to reference the steps: Recommended settings for Wi-Fi routers and access points - Apple Support

Kind regards.