Do your network segments look like this?
+-------------+ +------------+ / \__/ \
Internal LAN------+ Your Router +-------+ ISP Router +-----+ Internet /
+-------------+ +------------+ \________/
If so, your inner router must be misconfigured. You're right, MAC addresses are not passing a correctly set up router because ethernet is not routable.
So it doesn't matter whether that portal sits on the ISP's router or somewhere in the internet; in both cases, it shouldn't have access to your internal MAC addresses unless you somehow actively provide them.
If your ISP's Router (located at your home) provided WiFi to your internal clients, then obviously that would explain where it got their MAC addresses. Or if your router acted as a bridge (and especially if the ISP's router then offered DHCP to your internal LAN), that might explain it. Or if you installed a special app on each of your devices that sent the device mac to your ISP, that would also explain it. I think that in all other cases, your router is configured wrong; it's passing information it shouldn't.
If you're not sure whether your router is acting as a bridge, tell us the network addresses of your internal lan and the lan segment between your router and the ISP's router.