Discord only supports VPNs that have a User Datagram Protocol (UPD). You need to make sure your VPN (if you are using any) is compatible. This should fix the RTC Connecting stuck error on your Discord. Now you have to select the “Turn off Windows Defender Firewall (not recommended)” option in all three network types. From the newly opened window, click on the “Turn Windows Defender Firewall on or off” option from the left side. Press Win Key+R, and in Run Window type “control firewall.cpl”.
If you have not bought an additional firewall and if you are using the default Windows Defender Firewall, then you can easily disable it. You could try temporarily disabling both Antivirus and Firewall to check if Discord is working. Also, make sure to check your Firewall for the same. You need to check this in your Antivirus and whitelist Discord if it is so. But anyway some antiviruses do consider Discord as a treat and blocks it.
Go to your Antivirus and also the Firewall and make sure they have not blocked Discord for reasons beyond our normal understanding.