If you have a server ALWAYS use DHCP with it. NEVER use DHCP from the router and NEVER, EVER have them both on. I am very sure it was your firewall.
Just a note. This is not enabled by default. You will have to add the role in Essentials. I can't remember who, but someone PM'ed me about their network not working when they disabled the DHCP server on their router. It's confusing because in Standard, the DHCP role is auto-configured. In Essentials, you have to add it.
Essentials does not have this role installed by default. You can enable it. Check part 5 of the series. Just don't forget to disable the one on your router as Bert indicated earlier.
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