Again, I highly doubt there's any man in the middle attack going on. Yes your ISP router can be compromised but it's unlikely. You can change your ISP admin password if you wish (just remember to update your sticker on the modem / ISP router to reflect your change) also make sure your firmware is up to date or at least on auto update. Since you are using the Brume 2 and mango for WiFi you can also disable the ISP WiFi if your mango is covering your whole area.
As for a bad actor getting on your brume 2 they would go via the brume 2 interface, it's not related to the ISP router so there's no way of them getting from one to the other without knowing the brume 2 password or using an exploit.
Some logs regarding packets etc can show but that also can be normal.
Unless your a secret service spy I think your safe. It was probably an old client or some client you have forgot about that was appearing.