First you need to choose the router model. I have only tried TP-Link TL-WR703N, TL-MR3020 and TL-MR10U models. Here are the specifications of the devices:

• Atheros AR9331@400MHz MIPS 24K core
• 10/100 Mbit Ethernet port
• 4 MB flash memory
• 32 MB RAM
• USB 2.0 port
• micro-USB port for power

OpenWrt is a Linux distribution for small embedded devices like routers. What is the advantage of using small embedded device with OpenWrt and whsnbg as MQTT broker-gateway over the regular computer? Please see the pictures below. These pictures are the fragments of the network topology diagram from here: