Synology are offering free VPN Plus licenses to all Synology Router users in an effort to support businesses with employees working from home during COVID-19. The licenses don't appear to have end dates.
Number of licenses is dependent upon the routers you have purchased:
Device Model Maximum Client VPN Access License
Site to Site VPN license is also free: link
these $10 licenses are being given away on a perpetual basis. They do not require additional fees after September 30, 2020