My extender is connected with Ethernet cable in a 100mb/s network but the signal is delivering is only 20mb/s 

Im not connecting it with the router via wireless because the ISP told me that the 2.4 signal from the router will be alot less(around 40 or 50)  but the 5.0wireless  is +100mb/s.

i tested it, its true

Due to the fact that my extender only accepts 2.4 wireless, my only option is to use the Ethernet cable to get as much speed as possible, but 20mb is just too slow and i want to know what i have to do to fix this. 

Router and extender are connected with Ethernet cable, that is 3 meters long.

And the device im using is just 1 meter away from the extender, connected with wireless connection. 

Tested the 5.0 wireless signal from the router: 134mb/s

Tested 2.4 wireless signal from router: 42 mb/s 

Tested wireless signal coming from tp-link extender connected to router with Ethernet cable: 19 mb/s