So basically these days, no router or switch works with 9v or 5v, all such devices will contain dc-dc converters that convert the input voltage down to lower voltages like 3.3v and 2.5v or even 1.8v
DC-DC converters these days are in pretty much two big ranges : ones that support up to around 5.5v-6v and the ones that support more than 6v input voltage.
If the router designer chose a dc-dc converter that uses more than 5v, it's usually because higher voltage power adapters are cheaper to make and are more reliable. For example, if the router needs 10 watts, it can either get these 10 watts as 5v x 2A = 10 watts, or it can get them as 7.5v x 1.4A or 9v x 1.1A or it can get them as 12v x 0.85A
The higher the current, the thicker the wires must be between the power adapter and the adapter will be hotter and may go bad sooner.
So it's just more convenient or a manufacturer of routers or switches to just mass order 50k pieces of a 9v or 12v adapter and use it with 10-20 different models of switches and routers.
Now this being said, the manufacturer probably "optimized" the dc-dc converter to be most efficient at around 9v, but still be quite efficient at 7.5v and probably even 12v - think of it like gears on a car ... they may have optimized the router to be most efficient at 9v in : 3.3v out, or around 36% ratio. At 12v it would be 27% , at 7.5v it would be 44%
With 5v, you're talking about a ratio of 66% which is quite high. So the dc-dc converter may have a hard time producing the maximum power it was designed for, or may do that very inefficiently.
It's quite possible that the circuit was designed to produce 10 watts but with 5v input it may only be able to produce 6w but that much could be enough for wireless and one ethernet cable plugged in - if you plug more ethernet cables or you have more wireless devices, it could be that the power consumption goes above the threshold the converter can provide and you may get random reboots or disconnects.