How to keep agent ssh active?


I have linux and I try to keep my ssh session but everytime I reboot I have to set again the agent and password about router. Sometime, I'm still on the same session and it asks me the passphrase again.

I use that;

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519

But I'm looking for something more stable? On ubuntu and mint, I know we can check a box the first time it asks the passphrase and that way, the system remembers it

Use SSH keys.

I use pub keys and private.
With passphrase, I guess I can remove passphrase to be ok