Hi, I have Archer C7 router at home. Today I've increased bandwidth from my ISP to 300Mbps. On the cable I can reach 300Mbps.
On the network 2,4 GHZ I can reach 80Mbps. 
On the network 5GHZ I can reach only 25Mbps.

I'm sitting 4 meters from router. One 8cm wall between me and router. 
Test has been performed via speedtest.net website
This situation exist on 2x laptops and 2x Iphones (7+ and XR, both on the newest IOS). It is really strange that sitting in the same place the difference in speed is huge. I was trying to change the configuration - in section mode, I was trying to change from 802.11 a/n/ac mixed where I have 25Mbps, to modes 802.11 n/ac mixed or 802.11 ac only. In both cases when I've changed the mode I was not able to connect to the network. On the laptops I have error - unable connect to network. On Iphones I have error - wrong password, when I'm providing correct password I'm receiving the same error - please provide password. 5GHZ network is working for me only in mode a/n/ac mixed.

I'm affraid the router and is broken.Could you please advice how should I configure 5GHZ network to have highest speed - 300 Mbps?
I would appreciate your answer.