Lithium batteries cannot be connected in series if their capacities do not match exactly. Otherwise, the smaller capacity will be filled in advance while the larger capacity will not be fully charged. After several cycles, the voltage difference of the batteries will become larger and larger, and finally the entire battery will be discarded.
Actually, you don't need that many batteries. The power of the router is very low, only 6W. When the capacity of one 18650 battery is around 9W, you use two 18650s in parallel (in parallel). the connection does not need to match the capacity, it will automatically balance the current), then add a DC-DC conversion circuit to increase the voltage to 12V, and it's done.
Using two 18650 batteries connected in parallel brings great convenience to charging. You just need to buy a battery protection cardready-to-use lithium battery, then connect it directly to the DC5V power supply. No further action is necessary. .