Lagging issues on PS4

[#1]

I recently had an engineer out for a fault causing slow speeds. Since the visit one of the PS4's being used is suffering lag whilst playing online games. Other devices such as a separate PS4, laptop and phones are all fine. Any help or suggestions would be appreciated. 

[#2]

How are each of the devices connecting to the internet, are there any differences? Also what router is being used?

[#3]

Thanks for your reply. 

All devices are connected via wifi. 

The router is Plusnet Hub 2 (I think) lol. 

[#4]

Have you checked the signal quality around the area of each device’s location? Maybe a neighbour’s WiFi is interfering for the problem PS4?