Sorry to hear you are having issues.
First, Make sure you're using the correct key for Fences V3. Than, please check if you have not reach maximize activation count. You should try deactivate old activation using this guide : https://forums.stardock.com/486311/accounts-and-activation-support-faq#deactivateproduct.
Try those and if still fail, please raise a support ticket using the link below. Choose account management option. Include all information details. Remember to record down the ticket number for future reference if needed. Do not raise multiple ticket, that will make your ticket back to the end of the line. Send once and wait. Ticket will be answer by the order they send in.
Stardock Community Assistant.