Be sure the router is at its latest firmware. Also, the 2.4GHz band often clocks in lower on so many models. Use the 5GHz for speed.
As to 100 megabits, I fear that since you didn't opt for gigabit ports you are seeing the results. I'd try a newer dual core router with gigabit ports next.
I did this and it kicked the old 100 megabit port model to the curb.
I have 100Mb symmetrical contracted with Axtel, they installed a Zhone 2516 modem that through direct connection with cable from my PC to the Zhone gives me average speeds of 90Mb both download and upload, and through WiFi only 20-30Mb Download and Upload, so they tell me at Axtel that it is necessary to buy new equipment with 5G band to have the 100Mb over WiFi.
I purchase the TP-Link Archer C50 V3 from an authorized dealer with enough features to provide the above mentioned speed. I configure it with advice from Axtel technical support so that the Zhone modem changes "layer" and then the TP-Link already with cloned MAC can work properly.
I start to test the speed with speedtest.net. Through a cable connection from my PC to the Archer this gives me >90Mb of download and upload, confirming that the router effectively receives the 100Mb contracted from Axtel. However with Wifi test (my PC is 2.4G) it gives me about 30Mb of both upload and download, although sometimes it does give me >40Mb.
The numbers may seem "normal" for band 2.4, however a few months ago I had 50MB contracted with IZZI, and in all my devices (the same, including cell phones) I always had speeds of 40-50Mb with their Techcnicolor router band 2.4 Ghz, discarding from here that my computers or my wireless cards could be the problem at the physical/hardware level, besides the fact that the WiFi signal is good and there is no equipment that could interfere with the signal.
Continuing with the tests: My Galaxy S5 cell phone that has both bands on its wireless network card gives the following results consistently:
Band 2.4: ~20Mb download, ~50 upload.
Band 5: ~30Mb download, ~90Mb upload.
It can be seen that the upload speed is consistent with the band being used, however the download speed always remains slower.
Another device, a Galaxy S4 cell phone.
Band 2.4: ~35Mb download, ~50Mb upload.
Band 5: ~55Mb download, ~95Mb upload
During these days I configured the router, (followed the steps described here: https://www.tp-link.com/us/faq-1077.html), changing WiFi channels, WiFi security types, firmware update (https://www.tp-link.com/en/download/Archer-C50_V3.html#...), cable replacement, router position change, disconnection of wireless devices in my house, etc. and despite all this the router did not improve, the WiFi problem is still present.
All this I showed them is exactly the same as what I sent them to TP-Link technical support but they said: " the router will not give you more since the provider is not giving you what you have contracted, you must ask for the change of the Axtel modem. It's the only way to make sure you're actually getting what you agreed to. Greetings."
But personally for me that doesn't make sense, because I'm already receiving the 100Mb from Axtel when I connect through enternet cable my laptop to the router (and the modem too). And if I talk to Axtel they tell me that the problem is the TPLink Router.
I need to know if the problem is about the router configuration or if it is a physical problem and requires the router to be replaced, and how to clarify it with the respective company.