That's usually a sign of congestion.

Majority of home ISP users are on the download side.

How can I prevent this from happening again

an outdated modem from 2019

Just pointing out you answered your own quesiton. If you know you're on an old / unpatched / vulnerable router I would suggest getting one that is patched. There will always be things crawling the internet for exploits. Some just look but others may attempt stuff on home users.

Just my $0.02