Router username passwd not working

[w_i_k_i_d]

I have a Actiontec MI424-WR router.

I know its old. I need to go into the settings of the router.

Whatever username/ password I set it up last time is not working anymore.

I want to know why that is happening.

The only way I can access the Router settings is by pressing a hairpin into the back of the router and resetting it, then it would ask me for a new username/ password. I will make new ones. Everything is fine. 

Two days later I want to log back in and I can't with the same username/password ? Why is this happening ? 

[Edg1 · Accepted solution]

If I remember correctly the default user/password is admin/password. Verizon did an update for those routers and changed the password to the serial number for better security. So for example your password should be CSJDxxxxxxxxxx. The letters are caps. User name should still be admin.  What probably happening is that after you login the password eventually changes back to the serial number. 

[henry218]

Thank you @Edg1. Your directions helped me get into the settings.