Ads intended for one device reaching another device.

I have AT&T Wi-Fi gateway, model #5268AC.

A complete stranger visiting my house signs onto my AT&T wi-fi network for internet access.  I'm on my laptop searching for a door at Home Depot.  10 minutes later, the stranger begins seeing ads on their phone in their browser for that specific door at Home Depot.

I've searched around the internet for this situation, and it appears to either be A) Household ad targeting via the public IP Address of the router, or B) Un-requested incoming traffic to the router (the public IP Address), and the router doesn't know which port or which private IP Address on the network to send it to, so it goes to an unintended recipient.

A and B are somewhat similar, but one is intentional by advertisers, and one is maybe unintentional.  Regardless of which one, is there anything I can do on my AT&T router settings to stop this from happening?  Should I just change the firewall settings for that one device (phone) within the AT&T router settings to block all un-requested incoming traffic?

Also, I don't want this discussion to start going in the direction of like people being logged into the same google accounts, browser synching, all of those other typical reasons we might see ads on a different device.  That's why I specifically stated that it's a "stranger," as I've already eliminated all of those other possibilities.