I have been using my TP Link AX3000 as my WiFi router for a week or so now, it is connected to the ISP router (Plusnet Hub One) which acts as a modem via the LAN cable supplied with the TP Link Router, from LAN 1 on the ISP Router, to WAN on the TP Link Router. It used to work fine and I'd get the same internet download and upload speeds but with lower latency on the TP Link Router (which is what I originally wanted, for gaming). 
 

For the last couple of days however, the TP Link router has been very slow (~15mbps download) compared to the speed of the ISP Router if I connect to the wireless on that instead (70.5mbps) - what's the situation here? Why has the connection speed dropped suddenly for the AX3000 but remains good on the ISP router?

I bought the TP Link router to improve my internet but it just seems to be having the opposite effect recently, quite dramatically so.

Any ideas what's happening?

Here are some screenshots of my iPhone connected to each router - 

ISP Router (Plusnet Hub One): 

TP Link AX3000 Router: