The Keenetic Ultra II router only lowers the upload. Where to dig?

Question

Computer networks

I needed a fast Internet channel to work with cloud storage, and then the offer of 365 Mbps from Beeline just turned up. Internet beeline provides through L2TP.
Router: Zyxel Keenetic Ultra II. The best. Everything is plugged into gigabit ports.
But as it turns out, there are problems. The measuring tools were: speedtest.net, Yandex.Disk, a 1 gigabyte file posted on the provider's test page.
With a direct connection to the provider->computer, all speeds correspond to the tariff rates for all measurements.
When connected via a router, the download speed corresponds to the tariff, and the upload speed is no more than 100 Mbps. At the same time, the problem with upload appears only in real work (Yandex.Disk, Selectel cloud (there are no restrictions, 900 Mbit+ crawls from other providers) and others.) Everything is fine with speedtest.net.
The processor of the router is loaded by no more than 5%, almost all the memory is free, there are no extra packages. The latest release firmware is 2.06.
On the Beeline forum, I was advised to plug a switch in front of the router that should catch CRC errors. But I don't have a switch.
I will be glad to hints what else it could be and where to dig.

Answer 1

Hmm, strange, everything seems to be working fine. Try to look at torrents, maybe Yandex itself kills its channel.

Answer 2

change the MTU value in the L2TP connection settings
Most likely you have 1500 set, on the beeline forums it is advised to enter 1460 there