Hi Foxint,
1. What is the difference between ETH0 and WLAN0
ETH0 is the Pi's Wired Ethernet interface. WLAN0 is the Pi's Wi-Fi interface. If they're both active (i.e. showing a green tick in the Pi's Networks page) then your router will assign a different IP address for each one.
2. How would I assign an IP Address to the Pi via the modem?
I can't give precise instructions here, as every router manufacturer is different - you'll need to read your router's manual to do this. Look for something like "Address Reservation" or "DHCP Reservation", and enter the IP address you want for each of the Pi's network interfaces.
When you assign a fixed IP address you'll need to enter the MAC address for that particular network interface - the MAC address is a completely unique ID for every network interface. It'll look something like "a1:b2:c3:d4:e5:f6" (the exact numbers will be different, of course). With luck it'll be included in the list of 28 DHCP addresses that you mention - otherwise you can find it in the "###### Network interfaces (ifconfig) ######" section of the Pi's Debug page (about 2/3 the way down - there are separate sub-sections for "eth0" and "wlan0", and you'll be looking for the number in the line that starts with "ether"),
I hope this helps,
Peace and friendship,
englishtim