Hello Furry thanks for the quick reply!
Below you can find the answers:
1. Hardware version = A1
2. Firmware = 1.11B03
3. Region = Asuncion, Paraguay, South America
4. Wired or wireless? = Wireless connected - all 5 laptops + maybe a couple of smartphones
5. ISP service type = DSL
6. Modem Mfr. and model # = ZTE - ZXV10W300
7. ISP service dynamic or static = Static - public IP address - PPOE
Is the ISP modem bridged? - If the ISP modem has a built in router, it's best to bridge the modem. Having 2 routers on the same line can cause connection problems: Link>Double NAT and How NAT Works. To tell if the modem is bridged or not, look at the routers web page, Status/Device Info/Wan Section, if there is a 192.168.0.# address in the WAN IP address field, then the modem is not bridged. If the modem can't be bridged then see if the modem has a DMZ option and input the IP address the router gets from the modem and put that into the modems DMZ. Also check the routers DHCP IP address maybe conflicting with the ISP modems IP address of 192.168.0.1. Check to see if this is the same on the ISP modem, and if modem can't be bridged, change the DIR router to 192.168.1.1 or .0.254.
Example of a D-Link router configured for PPPoE with ISP Modem bridged: PPPoE Configuration on Router
8. Model link speeds = UP 512 KBPS - DOWN 2048 KBPS
May not be good enough for lots of gaming or SB may not be able to handle all the LTs online at the same time with such a low ISP speed.9. This only happens when all 5 laptops are gaming, if the devices are browsing or even downloading something there is no problem... as mentioned, when disabling streamboost it all works well, but the reason why the router was purchased is because of streamboost!
It maybe that since this router isn't actually officially supported in your region, that there maybe some SB updates that you may not be getting nor the game you play may not be included as something SB might manage. FYI> http://forums.dlink.com/index.php?topic=55110.0 You may need a router that is officially supported in your region:>>> http://www.dlinkla.com/detalle/familias/61#2122