How do I activate 5G on OnePlus 8 Pro?

You must have a network provider who provides the VoLTE/VoWifi service on your mobile plan . The VoLTE/VoWifi settings will be in your Sim & Network settings, if its not showing and you have a Network provider that supports it as well as a good 4G LTE signal then try the following to see if this will enable it.

Choose SIM & network. Choose Preferred network type. Choose 2G/3G/LTE/5G (Automatic) . The OnePlus 8 5G has now been configured for use of 5G networks.

This phone is unlocked and compatible with GSM + CDMA/4G LTE + 5G networks . With 5G, you can experience extremely fast downloads and streaming content up to 4K resolution. For additional connectivity, you can use Wi-Fi 6 (802.11ax) to access local networks and the internet.