There are several ways to wake up the server:
!!! For all this examples you need to config your Server-BIOS for WOL first !!!
1) Over a magic packet within your LAN (WOL - Wake on LAN) or over the internet (WOW - Wake on WAN)
2) Over an App like "Wake On Lan" (Android) - works with WOL and WOW https://play.google.com/store/apps/details?id=co.uk.mrwebb.wakeonlan
3) Directly, when you start your program (Plex, Kodi) on your PC - for this you can use a *.bat file which starts your client and then send the magic packet.
You can do this over wolcmd.exe (Windows)
If you use a Raspberry (like me) for Plex (or Kodi), you have to use the Phone-App because Plex doesnt have a WOL-function integrated!
Broadcast Example (within a *.bat File) for a Windows machine to send a WOW (Wake on WAN):
wolcmd 0A0A0A0A0A0A 199.199.199.199 255.255.255.255 66666
The 0As are the MAC of your server
The 199s are your IP on the Internet
The 255s are the broadcast adress
66666 is the Port where the WOL-Packed comes in. The Router must take it and forward it to port 9 (Firewall rule needed)
For a dynamic DNS entry replace the 199s with your dynDNS entry like xyz.dyndns.org (example).
The App can also remotely wake your Server over WAN (internet) but only with a propper config on your Router (Broadcast)
And here is an example for a *.bat file (with wake.exe) - this works only within your LAN:
d:
cd \
cd "000 - SUPPORT"
cd "WOL Tool"
wake FF-FF-FF-FF-FF-FF 192.168.x.x
cd \
cd "Program Files (x86)"
cd "Plex Home Theater"
"Plex Home Theater.exe"
When you execute the *.bat, it sends an WOL to the server and starts the Plex-Client.