MS Teams Accounts keeps being locked due to VPN

Our company just start using MS Teams to working from home, but we need to connect to company network using VPN. Our staffs sometimes connect/disconnect VPN while MS Teams application still log on, and when they try to re-login the account were locked. Some of our staff already unlock the account using security code via SMS, but in the same day the same account was locked again until "usage limit exceeded" when requesting a security code. Is there any guidance on how to prevent the account locking?