@ameed 

Ok, so the combined resistance of the 25m long cable is 6.6 Ohms. 

The voltage drop over a given resistance at a given current is calculated: voltage = resistance × current.

For your old router that equation would be 6.6 Ohm × 0.25 A = 1.65 V and for your new router 6.6 Ohm × 0.15 A = 1 V, but we have to consider that actual currents will vary as the router's workload changes (idling vs. lots of network traffic), especially peak currents can be much higher and hence the voltage would sag deeper.

I guess powering a 9 V device via a long extension cable is just not a good thing.

If you want you may do the same resistance measurement for the coaxial part of that cable (core and shield) to see how much resistance it has in comparison.

  @woozle 

I'm  not using solder, I'm using regular electric Tape to link the DC plug connector and making something like knots. 

Does that affect? 

  @ameed 

Yes, this will also lead to additional resistance being added to the overall resistance of the cable. 

Fixing a wire with knots or wrapping it around will only work for applications where there is not much of an electrical current, for example if you were to connect a passive antenna.

As soon as there is substantial current flowing, the wires will need to be soldered or clamped (e.g. via a screw or a spring-loaded contact).

  @woozle 

Core Resestense : 3.5 ohm

Shield Resestense : 2.0 ohm