Also how do you get the speed of transferring file? Which app you are using to transfer file?

Are the PC, PS4 and Iphone connected via Ethernet cable or wifi separately? If they are connected via wifi, you may check the wifi link speed on the devices, and try to change wifi channel and...

You may email tp-link support to get help.

How did you upgrade the firmware, via cable connection or wifi? Is there any power outrage during upgrading? You may try reset the router and see what happens.

Haven't tried this one. Here lists some models tested by TP-Link: http://static.tp-link.com/TPLink_20170419_Printer_CompatibilityList.pdf It includes the model EPSON TM T88III, but not sure about...

Can you provide a screenshot so we can check?