I am using To Do desktop app on Win 10 and it synchronises to my Office 365 account fine when working from home UNTIL I pull up my VPN into the office - it then says unable to sync! I can sync fine while in office so there must be something being blocked by the firewall config for VPN users but in order to get my company to address this, I need to understand what to ask them to open.
Dean's advice should help with this. We are using Exchange Online ports for sync and they are on the official list he linked to. Please let us know if you continue to have issues after whitelisting those resources.