@Shanu00 

Upgrade the device FW to the latest version described in this thread.

Watch for the correct FW region, whether US or EU.

For EU FW only a beta version is available, but you can use it without any worries.

When you're with the latest FW release, you'll be able to disable the OneMesh feature on the device if you're not using it.

Do that and see if that improves your wireless experience.

If after these actions the issue persists then try the suggested actions from this thread.

@Shanu00 

The router reboots itself immediately when a device tries to connect to its 5GHz band - is that correct ?

If it's not - describe the issue in details.

You can do the following to troubleshoot the issue:

Make notes of your router configuration and create a backup via Advanced=>Systems tools=>Backup & Restore.

Hard reset (method 1) the router and configure only the basic staff like Wi-Fi SSID and password.

Test if the 5 GHz band works properly.

if it does, then the problem is with the device configuration - re-configure the router from scratch (use your notes).

If it doesn't, then probably you have a HW issue - contact your local support to be advised further - meanwhile restore the router configuration from the backup file.