Router TP-Link TL-WR1043N/ND v2
OpenWrt Chaos Calmer 15.05 / LuCI (git-15.248.30277-3836b45) Kernel Version 3.18.20
Connecting Ipoe. By cable from the router 100mbps download and upload, via wifi ~ 15mbps download and upload. According to the specification, the wifi router supports 100mbps
I made sure that the channels do not intersect. Tried all possible wifi modes.
I read that some kind of proprietary hwnat is needed and that it is not available in binary form. But they talked about it 5 years ago. Maybe hwnat is no longer relevant?
What to do ?
Since your device is still supported, but Chaos Calmer is not, upgrade to the latest firmware.
Direct sysupgrade from 18.06 (or earlier) to 21.02 is not supported. To upgrade, you should backup your configuration, install 21.02.0, and then manually re-create your configuration.
Hardware NAT is not supported in OpenWrt for the TL-WR1043N/ND v2
Depends on what the actual issue is.
But in order to get support, you'll need to upgrade to a supported version of the firmware.
Is your client a 1x1 radio? Practically about 45 Mb actual throughput is the limit on 2.4 HT20. HT40 is only useful if you have absolutely no other signals from neighbors in the 40 MHz block.
But 15 is rather low unless the client is somewhat far from the router. You can look at the Tx and Rx raw bit rates and MCS numbers on the main status screen. Again actual usable throughput will be half of the raw rate shown with MCS.
Like @anon89577378 mentioned do upgrade or reflash with care and without keeping settings since the architecture changed from ar71xx > ath79!
With 20Mhz dated 2,4GHz N-wifi hardware ~70Mb's download should be no issue.