Most ADSL routers make use of 12V power supplies, providing around 1-1.5A. It’s important to note, though, that this takes into account running the router at maximum capacity on all ports and antennas and supplying power to an external device such as a hard drive. This means that, in practice, routers use considerably less power than the power supply is rated for. I took advantage of this to run my router off of a 5V 2.1A power bank USB port. You can also run this solution off of a laptop USB port.
To convert the 5V supply from the power bank to the 12V supply necessary to run the router I used a boost converter, which essentially trades Amps for Volts (the power output in Watts essentially remains the same, minus a small conversion cost). The 2.1A supply times the fraction of 5V divided by 12V gives a maximum output current of 875mA, which turns out to be sufficient to run the router with minimal devices and traffic.
If you suspect that around 800mA would be sufficient to run your router, you can do the same, either by using a bare converter or a ready-made solution for around the same price. When using the ready-made option, make sure that the existing barrel connector fits the receptacle on your router, as they come in different sizes.
Something else to keep in mind is that some routers rated for 12V can run directly on a 5V supply – so that might be worth trying if you’re willing to take the risk, although it might result in data loss on the router’s internal storage. A USB to barrel connector cable can easily be found online.