Hello,
I have this problem with this router on my internet provider. I have a PPPoE 500/150 mbits/s link and in cases when link dropped and reconnect, after that the internet connection is unstable. I cannot acces some websites or i have very slow acces. For example if i want to search on google, it will eventually open after about 20-30 secconds. All download starts and drops after 5-10 secconds. Ookla speed give me error. After i restart the router everything work fine until a new PPPoE recconect. I can reproduce this problem even if i manually disconnect and reconnect from router web interface.
This problem not apear if i disable hardware NAT. The problem persist with a seccond/replaced router- the same version V 4.0 and confirmed by TP-Link support from my country. Until now ( after two monts) nothing comes from TP-Link, no word, no firmware, nothing...

Today i discovered that V5 has the same problem...