It is perfectly possible and safe, you can use a relay, in which case the sample fails by the AC adapter, use the battery as a backup source.
To do this you simply connect the AC source in parallel with battery because if the battery were to fail or to download the source to give energy Peplink.
To be even safer would be good to use some kind of circuit that activates when the battery fails the AC source, the problem is that this circuit has to be done right, otherwise Peplink is shut down during this exchange of energy source.
You can go in any electronic or forum on the subject and ask about this circuit.
But worth a watch, the energy source must be the same for battery voltage when switching sources do not harm the input circuit power Peplink. What happens is that it works well when you turn on and off it at a certain voltage, but during the operation of the product of the voltage changes nothing for other energy source may be some damage to internal circuitry, since it would be a sudden change of energy.
Remembering that any damage to equipment caused by an external circuit inefficient or defective shall not be covered by warranty.