The 600 V1 is horribly underpowered so it's believable there's a hard 400mbps limit but there are a couple of features that cripple performance to disable.

1: make sure NAS Boost is enabled under Advanced\NAT Forwarding. This will also disable QOS and data monitoring, both of them also hit data throughput hard and QOS doesn't even work.

2: reduce logging as far as possible. I couldn't believe how much it hits performance. Never seen anything like it on released hardware.

If you still see only 400mbps, you're going to need a better router than the V1. It's a budget device with premium pricing.

@JerryAtrick After enabling NAT boost and disabling logging, 400 mbps is still the max speed that I can get.

What other router from tp-link, OneMesh compatible and 4G could do the trick?

Thank you very much!

@epyblast Let's try again without the site trashing my words...

Can't comment on other TPLink products, this POS will be my 1st&last product from them. No support for a barely functional router is a red flag to avoid them.

One last thought: have you tried disabling the mesh and testing again. Shouldn't affect a wired LAN port but after a year of MR600 I can believe they would somehow manage to import mesh WiFI bandwidth reduction (it's not tri-band) to the LAN connections.

@Kevin_Z 

I connected my laptop to the MR600 via ethernet. No wireless clients. No mesh extenders. MR600 is connected to the main router. This is the result:

If I do a test in the MR600, this is the result:

I will try the factory reset now, then show you the results.

Edit: My ISP upgraded my FTTH connection, so now I have 800/800.

@Kevin_Z Ok, but what is the max speed of the WiFi interface of the MR600?

I mean, the ethernet appears to work ok, but the WiFi interface is slower. What is the speed limit of the WiFi interface of this router?

WiFi bandwidth 300Mbps over 2.4GHz, 867Mbps over 5GHz

You won't see that in real life use, even sitting next to this router. Use it wired or replace it.