Changing max vdsl sync speed

I am trying to change the max sync (download) speed in my VDSL connection on a BT HH5A with Openwrt18.06.8. I want to decrease a bit the max download speed (Data Rate) in favour of connection stability. I have spoken to my ISP and they do not decrease it for me, although I have proved that this is going to be beneficial in my case.
I have seen that Technicolor TG799VAC uses an openwrt variation and the following set of commands serves this purspose:

uci set xdsl.dsl0.maxaggrdatarate='300000'
uci set xdsl.dsl0.maxdsdatarate='250000'
uci set xdsl.dsl0.maxusdatarate='50000'
uci commit xdsl

However when trying to do this on my BT HH5A, I am getting "uci: entry not found". Is it because no such dsl profile currently exists or is it because the generic Openwrt version does not support such command (i.e. It is Technocolor specific)?

In the following forum, a guy has managed to do it on a Fritzbox7590 which uses Lantiq chipset too.

Finally, just to confirm, is this set of commands going to change the actual DSL sync speed (Data Rate in the DSL Status) or is it just a throughput limit imposed on the dsl0 interface?

Afaik the TG799VAC does use a Broadcom xDSL chipset (BCM63138) and your HomeHub is using a Lantiq xDSL chipset. I guess those xdsl commands are broadcom specific.
Anyway I'm also using a vdsl modem with BCM63138 chipset and those commands doesn't have any affect on my linespeed. It could also be profile (DSLAM min rate) or linecard depended but in my case it doesn't work and my vdsl profile (min rate) would allow lower speeds than my current Syncrate.

Fritzbox 7590 does use a modern Intel xDSL chipset so it can't be compared to the old lantiq one which is present in the BT HH5A.