wifi Phones not working properly on 5GHz wifi

[Hulster_Hulse]

I have recently discovered that an iPhone 7 (running iOS 11.4.1) and a Samsung galaxy s7 (running Android 8.0) are near unusable on the 5GHz wifi band. They both get bellow 1Mbps and once the speed test (ookla speed test app) has run for a few seconds they both get up to around 3-5 Mbps but the upload will always go to around the maximum of my wireless speeds (20Mbps). However, my old iPhone 5S (running iOS 11.4.1) will get 70-80Mbps on the 5GHz wifi band so this leaves me even more confused as to why 2 newer phones are getting significantly worse speeds on the wifi band that they prefer.

I have run speed tests on all 3 devices on both bands and have provided screenshots below.

I have also provided screenshots of my router's (D-LINK DSL-3782) settings.

 

iPhone 5s speed tests

https://imgur.com/a/Q9lxZA3

iPhone 7 speed tests

https://imgur.com/a/igwOzPX

Samsung Galaxy S7 speed tests (low battery but was charging)

https://imgur.com/a/hZLcbaG

Router D-LINK DSL-3782 settings

(imgur wouldn't let me upload for another 55 minutes so I've attached the images below)

 

Any help is appreciated

 

Thanks

 

Edit: I have now discovered that an iPhone 6s (running iOS 11.4.1) is also having the same issue