The best choice is to switch off the 5G on your router.
Possibly even the 2.4G will be stronger afterwards.
You can test that with WiFiAnalyzer (open source), the one that asks the minimum permissons on your device.
Check signal strength of 2.4G before and after switching off 5G if you're curious.
The other choice is to goto settings->wifi->cogwheel->saved networks
Then tap on your 5G and tap forget.
Your phone cannot connect to 5G afterwards because it doesn't know the password.
It will choose 2.4G
