Hi HaoTran5,
Thank you for reaching Microsoft Community. I'm sorry to hear about the issues you are having in connecting your external monitor to your Surface Book 2. I understand how frustrating this is since this could really affect your productivity.
Does it loose connection when you leave the device idle or it also happens even while you are using the device?
Kindly try first to open Device Manager, and expand Universal Serial Bus controllers. Double-click each entry then check if it has a Power Management tab. If yes, go to the said tab and uncheck the box next to Allow the computer to turn off this device to save power.
If still the issue persist, this would seem like a loose connection on the USB-C port of your Surface Book 2. Kindly check and make sure that there are no dirt or dust on the port itself. If still the issue persist, I'm sorry but please contact our live support team to check for options available and for further assistance. You may contact them through the numbers listed in Global Customer Service phone numbers (microsoft.com) under For Business users. This works with Home users also, just select Surface then Home as a type of user in the IVR prompt. You may also reach them through this page: Contact Us - Microsoft Support and use the details or entries given here: I need to talk to tech support - Microsoft Community. Please note: For USA region, they are available from 6:00AM to 10:00PM Pacific Standard Time.
Kind regards,
Marrion