Comtrend wap-5813n too low tx/rx rates thorugh wifi

I've just recovered an old Comtrend WAP-581n router I had configured for a Movistar Fiber line. The idea is to set up an additional access point to access to internet from other places of my home that have no good quality signal now. I've installed OpenWrt 18.06.2 in my old Comtrend and connected it with a cable to my main current router. Then I've associated the lan network to the wireless:

The TX/RX rates that the Comtrend supports in theory would reach up to 300Mbps... through the LAN cable is receiving up to 60 Mbps...but if I measure it through my mobile or any other device connected to the "OpenWrt" SSID, I only get up to 6Mbps download and 5-6Mbps upload… Any idea why it's not giving all the bandwidth through the WIFI? Any configuration that should I look at?
Thanks in advance.

802.11bg = max theoretical rate 54 my friend

Please do some basic research and reading.

In a giant box at the top of the page;

Devices with Broadcom WiFi chipsets have limited OpenWrt supportability (due to limited FLOSS driver availability for Broadcom chips).

Broadcom has not released any FOSS drivers. Broadcom doesn’t support open-source much at all.

Limited means: 2.4GHz only b/g available, and 5GHz doesn't work at all. There might be some exceptions, but the general rule is as written before. For details see Unsupported features

And think twice when purchasing broadcom.

Thanks for the info. Too many things to take into account in just 2 days. But I understand now. I'll see if there is a way to use this router... or just buy a new one (no Broadcom :wink:) .