The only way I can possibly help is to google the answer to the question
asked in the title (which wasn't necessarily asked in the body).

You have an older Android 8 so the older articles might still be useful.
First hit (turns out to be a shill - but it begs a question)...

*How to Clear DNS Cache on Android Devices* https://www.trickyworlds.com/how-to-turn-off-suggested-posts-on-instagram/

As the OP had predicted, these are mostly about clearing web browser cache.
But at the bottom was information about 'system' settings.
Settings > Wi-Fi > {SSID} > Modify network > Advanced options
IP settings > Static > {change DNS servers manually}

Drat. That's not about clearing cache - but wait - if you _change_ the DNS server, does _that_ (in and of itself) cause the DNS cache to be flushed?

At the end of that article was the payback shill for this appl
*DNS Changer: Mobile Data, WiFi* by BGNmobi https://play.google.com/store/apps/details?id=com.burakgon.dnschanger
"The DNS Changer change�s your device�s DNS address"

I found plenty of DNS Changers which are both ad free & gsf free, where I'll just list the one on top of a filtered search based on being
free, ad free, gsf free, over a million downloads & rated highly
(since all these "DNS Changers" seem to do is muck with settings).
*DNSChanger for IPv4/IPv6 * by Frostnerd.com
free, ad free, gsf free, rated 4.6 of 48K reviews with 1M+ downloads https://play.google.com/store/apps/details?id=com.frostnerd.dnschanger

The question this brings up is whether simply changing the Android system networking DNS servers will "clear" the previous existing DNS cache?

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