If you cannot connect or re-connect your Bing source, try the following:
Ensure that multi-factor authentication has been enabled for your Microsoft account.
Connect a new source or reconnect an existing source in Funnel.
Create a new set of credentials. You should be prompted to enter an email address, however if you're not prompted then sign out of www.bing.com or use your browser in Private/Incognito mode, and try again.
If Microsoft states that your account exist both as a "Work or school account" and a "Personal account", select the option that was used when you got access to Bing Ads. If you don't know which one to use, pick "Personal account" and if that doesn't work, try "Work account".