Wi-fi "weak security" warning

I just got an alert from Xfinity that I was exceeding my data allowance on my home Internet. This is the first time I can recall getting this message, even though I've not been using the Internet differently than I normally do. I also noticed that downloads were unusually slow. So I started to wonder if some unauthorized person might be tapping into my Internet Wi-Fi signal.

I went to the Xfinity site/app for instructions, and I saw that Xfinity was part of a data breach not not too long ago, so that added to my suspicions. I changed both my Xfinity password (two factor authentication) and my Wi-Fi password. 

However, when I went to change the password on my iPhone, in Settings > Wi-Fi, under the name of my Wi-Fi network it says I have "weak security."

I don't understand why Xfinity, when they came over to my house to install my router and Internet, did not set it up with strong security in the first place. Has the security protocol changed since then?

 I tapped on the info button next to the warning on my iPhone and a long webpage came up now I'm trying to get a complete education in Internet security, but it seemed apparent that I needed to contact  Xfinity on this. 

I did so by chat and they said it is done automatically and continuously at Xfinity headquarters and they always set all their modems and accounts on the best protection from their end.

I guess I just need confirmation that it’s okay to disregard this iPhone security message.