iOS 14 WiFi 5GHz issues
In iOS14, there is an additional network setting that offers extra privacy/security by providing a private wifi address individual to each wifi network you connect to, which is particularly useful when connecting to public wifi networks. However, there have been reports of connection/performance issues when using private wifi addresses, particularly with wifi networks that have multiple wifi access points.
So as a troubleshooting step, you may wish to disable the private wifi address for the particular affected wifi network. You can disable the private wifi address by going to Settings&gt;Wi-Fi and press the ⓘ symbol of the affected wifi network (when you are connected to it) and midway down you will be able to disable the Private Address. Doing so will revert the wifi address to the device's default wifi address but only for that wifi network. This won't affect other wifi networks you have saved/connected to, but you can individually disable the private address for those too, when you are next connected to them. I'm not sure if this will fix your issue, but it is worth trying out.
Additionally, is it affecting other 5GHz wifi networks or just one specific 5GHz wifi network? It may have something to do with the 5GHz channel band the wifi network is on. If there's lots of other wifi networks (like in an apartment block) on the same channel band it can cause signal interference which is particularly worse on 5GHz networks than on 2.4GHz networks.
So, if it is just affecting the single wifi network, then it may be worth checking whether the channel band is on the same channel band as other nearby wifi networks and try change it to one that has the least amount of wifi networks on the same channel band. Hope this helps!