Why does the ASUS RT-N18U router cut speed over Wi-Fi?

Question

ASUS

Hello.
Initial data:
- ASUS RT-N18U router with the latest official firmware
- the declared speed is 80 Mbit / s from the provider, which is observed on the PC when connected to the router via a twisted pair cable, that is, it makes no sense to contact the provider.
- connection by type "Dynamic address"
- LG K10 LTE phone (Xiaomi Redmi 4A was also tested), for which speedtest does not give out more than 40-45 Mbit / s, that is, the speed is cut twice as stable.
What I tried:
- change the channel width
- supported wireless network standards
- switch different channels
- Tomato firmware, which was praised by many.
- countless more attempts to change settings
None of this significantly changes the situation.
The technical support of the router assures that this is the ceiling for the phone, but everything is fine with the router.
But on the Internet you can find many cases of such problems with ASUS routers.
There is also an iPad Mini, it produces even less.
It is not possible to check on another device (for example, a modern laptop).
Question: is it worth changing the router or is it not the case, but really the phone, and the other router will have the same situation?
And could there be such a good reason for a return under warranty?