Welcome!
I have no answers, but more questions
Could you please post the following data (mostly the output of commands run on the router via ssh)
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cat /etc/config/sqm
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tc -s qdisc
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SQM_DEBUG=1 SQM_VERBOSITY_MAX=8 /etc/init.d/sqm stop
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run a dslreports speedtest according to https://forum.openwrt.org/t/sqm-qos-recommended-settings-for-the-dslreports-speedtest-bufferbloat-testing/2803 and post the link to the results here
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SQM_DEBUG=1 SQM_VERBOSITY_MAX=8 /etc/init.d/sqm start
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tc -s qdisc
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run another dslreports speedtest and post the link to the results here
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tc -s qdisc
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Copy and paste the content of the "Share Your Result" box from https://www.speedguide.net/analyzer.php
That way we might be able to see what happens and why...
EDIT: fixed superfluous spaces, sorry for the confusion...