Wndr4500v2 wifi speed problem

Goodbye to all

I'm new and I hope you can help me.
I've a fttc connection 100/10.
Until now I couldn't test the wifi speed, cause devices hardware limit.
I've bought a Samsung s7 and I made speed test.
All my devices support 802.11n in 2.4ghz and 5ghz both.

My smartphone can't reach wired speed. I tried with another s7 and the problem is the same.
I use an extender (willy 0.2) that used by pc reaches the wired speed.
The same extender with xbox360 reaches 1/3 broadband connection.

The ports on router are correctly opened for the console.
Finally with wifi network analyzer app I found link speed at 54mbps, so my router limit the speed in 802.11a.

I set 2.4ghz and 5ghz networks to 450 mbit, removed 20/40mhz consistency but it's always limited.
I tested with only smartphone connected by wifi, but no luck.

Can I force router use use 802.11n on 2.4ghz and 5ghz or I'm doing anything wrong?

Sorry for my English.
Thanks in advance for any help.