If you are presented with a blank screen or you find the simulator is missing in MakeCode, you may need to configure your firewall to allow specific websites. Here is a list of requirements for each editor and tool.
microbit.org and subdomains (*.microbit.org)
readthedocs.io and subdomains(*.readthedocs.io)
readthedocs.org and subdomains(*.readthedocs.org)
as above and including
*.pusher.com (enable web socket connectoins if possible)
micro:bit classroom will use WebRTC if possible
If you are using a proxy server within a school environment, you may need to exclude the scratch link URL and port from using the proxy
This is set on the client and not the firewall or proxy.
How to set up an exclusion depends on how the machine's proxy settings have been configured in the first place. This is often done through Windows Group Policy, but may instead be done through a WPAD or PAC file or configured manually on each workstation. (Source: Opendium)