Linksys router 2.4 GHz & 5 GHz vs internet speed

Dear Experts

I am on ADSL max speed provided by telcomm is 25 Mbps yes 25,,,,,you can increase speed by paying more,,,,this is the issue
I have my Linksys router ACS1900 V2 connected , this router has two bands 2.4 and 5 both are configured.

(distance test in the same room where router located) I did test when I connect only one device my iphone 6 or Samsung Galaxy S7 mobiles to 5 GHz and 2.4 GHz speed of 2.4 is 1/3 of 5. 2.4 Ghz showing speed 6 Mbps while 5 GHz showing 18 Mbps,,,,,,,,,,,,,,,,,,,,why ?

I am pretty sure both mobiles have wireless of n standard or height right ? means they can handle average speed way more that ADSL speed which is 25 Mbps

(distance test in the same room where router located) I did the same test on my cousin house which he has cable internet speed up to 100 Mbps not ADSL technology , still 2.4Ghz is 1/3 of 5 Ghz,,,,,,,,,,,,,,,,,,,,2.4 Ghz showing speed 30 Mbps while 5 GHz showing 80 Mbps

To my understanding average speed for 2.4 Ghz on wireless n standard should be height than 25 Mbps right ?

Thanks