@ThirdColt 

 

you would need a way to ensure that the voltage stays at 12 volts. When your batteries are charging, what is the voltage? May be  ~ 14.3 volts. And when the batteries are drained to less than 12 volts it can cause issues. 
 

Others have done it, but I suspect the router life may not  last as long as when using the adapter.

@ThirdColt 

 

Most 12V units will operate @ 13.8V, charging will usually peak @ 14.2V

A simple way to keep the voltage @ a constant voltage is to use a Voltage Regulator, or a DC-DC Buck/Boost Converter which will do say 3.2V - 32V IN & 1.25V - 30V OUT, check out EBay they are very cheap to buy, just must sure you get one larger then the current (Amps) you require, say double or more.

 

Here is a solar powered unit  https://community.tp-link.com/en/home/forum/topic/272342

 

 

John vk3hjq

You can't use a buck converter. You have to use a SEPIC (or buck/boost) converter.

@Datman. 

 

yes a SEPIC - "Single Ended Primary Inductor Converter" that acts both as a Boost & Buck converter that will step Up or Down depending on the input voltage.

 

John vk3hjq

@vk3hjq 

 

Yes... that would work, but not much if anything is gained if any by using a SEPIC over using the AC/DC adapter that came with the router. I would assume if he had an internet connection, he woud have shore power.

@ArcherC8 

 

It sounds like a 12V supply only & the 240V is via a 12 V - 240V Inverter by the look of it, otherwise yes,  just run it off the 240V - 12V power supply supplied with the unit.

 

John vk3hjq