Micklest wrote

I have read that it might be the type of cable included with the router.

I doubt that it is the Ethernet cable. A CAT.5e cable can transfer data with speeds up to 1 Gbit/s.

To find out where the bottleneck is, much more information would be needed about your setup for tests, e.g.:

- speed of your WiFi adapter in your PC - does it support 802.11ac? Is it capable to meet the speeds of an Archer C50?
- negotiation of your PC's WiFi adapter with the Archer C50 - which speed does it negotiate?
- use of frequency band - Which band does it actually use (for example, the 2.4 GHz band can only achieve ~150 Mbit/s data rate over a 300 Mbit/s radio frame rate)?
- settings of the Archer C50 - what settings for both frequency bands do you use?
- environmental influences in the WiFi cell at your place - how many APs are around?
- actual WiFi speed between PC and Archer - what rates can you achieve?
- speed tests - which one do you use? For reliable results I would recommend dslreports.com.

There are a lot of things to consider if you want to do serious speed tests over WiFi. You also should first test speeds between the PC and the Archer and ensure that all requirements listed above are met, before measuring speed of the whole path from the PC to the ISP.

Micklest wrote

In other words, call the technician.

Not necessarily. But if things seem to go wrong, even a technician has no other change to find out more by looking at the setup step by step as outlined in my first comment.

Most often it then turns out, that the expectations are wrong in the first place.

If you have a router, which is able to achieve 300 Mbps in 2.4 GHz band for wireless radio frame speed, you will be able to use up to nearly 50% of this speed (~120 to 140 Mbit/s) for data frames. This is - beside the half-duplex nature of radio communication - mostly caused by the nature of TCP/IP communication, which 50 years ago was designed for data transfers through reliable cable connections, but not for radio connections. It therefore has nothing to do with the router.

Beside of this, you never ever can get more than ~120 to 140 Mbit/s over 2.4 GHz with modern modulation techniques at a channel width of 20 Mhz (requires 4 free and otherwise unused channels in the 2.4 GHz band). However, you can reach much higher data rates by using the 5 GHz band or by using combination of the 2.4 and 5 GHz band together with other techniques - if your PC supports it.

Anyway, achievable bandwidth not only depends on the router, which in most cases uses very specialized and high-end radio components and techniques, but it also depends on the radio (WiFi adapter) in your PC, which - for reasons of production costs - most often don't match the quality of radios built into WiFi routers.

First of all, I'm not from TP-Link. I'm just a user like you, who wanted to help.

Second, english is not my native language. Yes, a foreign language is my native language.

Third, I wonder why you didn't provide this information in your first post: if the modem's LED blinks yellow, this means you are using a 100 Mbit/s device. And no, I don't have all the specs of any TP-Link router in my head, but a look into the specs shows: you did use a 100 Mbit/s capable router instead of a Gigabit model. And then you blame the producer of those devices?

Lesson learned also: I would have suggested to hire a technician not for troubleshooting, but for selecting the correct device before buying it.