I've got Gigabit fiber and I recently decided to try and replace a couple of TM-AC1900 (flashed to RT-AC-86U) routers hoping that I'd have a more seamless mesh experience in the house and increased speeds. The good part is I don't lose connection in certain parts of the house anymore, the bad part is my speeds on the secondary W3600 aren't what they should be even though it has a direct ethernet connection to the fiber exactly the same as the primary W3600.

I wanted to keep the TM-AC1900s connected to basically use as access points since the W3600s don't have enough ethernet ports.

All tests done with speedtest.net

I've tried connecting everything a couple different ways both with and without the TM-AC1900s involved:

1- Both TM-AC1900s connected directly to fiber via ethernet. Both TP-Link W3600s connected to corresponding TM-AC1900s on opposite sides of the house via ethernet - basically sitting right next to each other.

Downstairs desktop computer connected to TM-AC1900 via ethernet gets 250Mbps down and 130 up

Standing 5 feet from downstairs TM-AC1900 and TP-Link W3600 Iphone 12 gets 600 down and 400 up (great!)

Upstairs desktop computer connected to upstairs TM-AC1900 via ethernet gets 270Mbps down and 275 up

Standing 5 feet from upstairs TM-AC1900 and TP-Link W3600 Iphone 12 gets 130 down and 75 up (wtf???)

2- Both TP-Link W3600s connected directly to fiber via ethernet. Neither TM-AC1900 connected to anything.

Downstairs desktop computer connected to TP-Link W3600 via ethernet gets 800Mbps down and 610 up

Standing 5 feet from downstairs TP-Link W3600s Iphone 12 gets 600 down and 390 up

Upstairs deskstop computer connected to TP-Link W3600 via ethernet gets 270 down and 275 up (???)

Standing 5 feet from upstairs TP-Link W3600s Iphone 12 gets 160 down and 130 up (seriously?)