The speed drops in the external network when connected via WiFi, what could be?

Question

WiFi

Hello.
There is an Internet provider with a channel up to 70 Mbps (PPPoE) and a Zyxel Ultra II router, the computer has TP-Link T6E.
The computer on Windows 8.1 is connected to the router via WiFi 5 GHz network, has a stable signal.
The speed in the local network (within the apartment) reaches 200-400 Mbps via WiFi (5 GHz) when copying files via FTP or from local resources (SAMBA). This suggests that there are no connectivity problems between the computer and the router. The problem is that if you upload or download files from external resources (Hetzner, OVH, Agava), then the speed drops to 10-20 Mbps. Here, many will think that the problem is in the Internet provider, but no.
If you connect the same computer to the router via a 1 Gbps cable (that is, traffic also goes through the router) or make a direct connection on the computer, then the speed in the local network increases even more (if the connection is through the router), and downloading and uploading files in the external network increases to 70 Mbit / s (that is, it becomes at the rate) and there are no problems.
There is also a Windows 10 laptop with a TP-Link T4U USB adapter. There is exactly the same problem.
The question is, what can it be and who to contact?

Answer 1

Judging by the description, the router does not have enough performance to route traffic between Wi-Fi and the Internet.
The decision depends on your capabilities. I would start by replacing the stock firmware with OpenWRT

Answer 2

Wangyu that sags percent. Or overloaded, or overheats when working on a wafer.
I would first put DD-WRT (or openwrt if you have experience). I looked at the info in the statuses in the load. And in general, their optimization is usually more interesting.