Did you try to remove the network profile on your S5 and reconnect with the password? If it still failed to connect, what's the error message?
You may try to disabled Wi-Fi security to see if it works. If not, you'd better reset the router to factory defaults and check if it will connect successfully.

I have bought an Archer D7 this WE and upgraded to latest firmware. I do experience the exact same issue.
All my other devices, computer, tablets, phones (iPhones & WindowPhone) are connecting properly whatever it is the 2.4Ghz or 5Ghz wifi network used.

The Samsung Galaxy S5 is well connected via WIFI and icon of wifi connected show up with arrows showing there are exchanges of data with the router.
I do see on the Router that there are some packets exchanged with the S5 phone however, any app on the phone using internet does not go through and result in a no internet available.

I do have a 3G WIFI modem and connecting to it works perfectly as well as it works well when connected to WIFI at work.

There is definitely something going wrong between the S5 and the Archer which I cannot figure out. I have checked all WIFI settings on the archer D& and all is properly configured. The fact all other devices work except the S5 means the S5 device has an issue with communicating with the D7.

Any help on this will be good.

Cheers
Mysteex

Can you login web interface of D7 on your Samsung phone?
As the router is working with all other wireless clients, try to contact Samsung for some advice.
If you don't mind, disable Wi-Fi security on the router and see what happens.