The server used for the test is hardcoded into the router so you can imagine if that server is down or unavailable that it may cause issues so it won't always be possible to complete the test. It is normal for it to disconnect all the devices from the internet though, it's so nothing else effects the test. Follow this guide https://support.netduma.com/frequently-asked-questions/legacyfaqs/test-your-ping/ while downloading & start with 95% for Congestion Control (set to Always), check results, decrease by 10%, check, decrease by 10% etc, until you get to a value that is pretty good & then try 5% either side of that value to see if it can be improved. Download & Upload on Congestion Control don't have to be the same value & you may have a better experience with differing values. This will be a good alternative to what you're trying to do.
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