Wi-fi connected but nothing loading on 2 of my Samsung S6 phones

[AC Question]

So, the strangest thing just started happening. Yesterday both of my Samsung S6 phones stopped connecting to the Wi-fi correctly. The phone shows that it's connected, the phone shows as connected when I look in my 'connected devices' list on the router, but nothing loads. I have tried: resetting the router, forgetting the wifi setting on the phone and re-entering it, clearing the system cache on the phone and setting the ip address to be static on both the router and the phone, and updating the firmware on the router.

Both of the Samsungs are running Android 6.0.1 unlocked. I also have a OnePlus 3t that is running Android 7 and it's fine, so I'm wondering if they need a firmware update but when I go try to get a pull update, it says there isn't anything (security patch Jan 1 2017).

I'm at a loss - the phones belong to my kids and they primarily use it for stuff like Instagram etc on wifi from home and this is killing my data plan.

Does this seem like a firmware thing? It seems so strange that both phones stopped working at the same time - and even if it seems like flashing the phone is the only option, how can I even do that without a wi-fi connection? I downloaded the G920FXXU5EQAC build to my laptop but don't know where to really go from there (their phones are unlocked G920F from Europe, we are living in Canada now)

Any help would be hugely appreciated!