Routers, Roku and guest accounts

[Julie1525]

We have 2 rooms listed, and each has a TV with a Roku stick. Plus we have a TV in the living room (no Roku). I'm not very technical. If 3 TVs are using wifi to watch anything, or tablets or phones, which router is going to be my best bet?  Currently, we have an Asus AC750 dual band router. It's about 4 years old and has served us fine. But will we need to upgrade to a better router, or does it depend more on the internet service we have?

Also, I'm concerned about the security of our internet account. I did some searching on how to set up a guest account, but to be honest, it was all Greek to me. Is there a tutorial anyone knows about that might walk me through it? Or is that dependant upon internet service + router + Roku/Amazon Prime/Netflix?

Am I being really silly to try to provide all this?

Julie