As long as you keep SPI firewall enabled the router will prevent users from gaining access.
If you feel that having a static IP address is not safe, I would recommend seeking out a ISP that changes or uses DHCP IP addresses and changes them every so often or ask your current ISP to change your static IP address every so often and let hem know that you have concerns about the and request that the IP address be change every so often for your peace of mind.
Good Luck.