Your router may have logs which they can check to look for issues, so could be that.
I've seen some routers where it displays graphs breaking down the types of usage (Web. FTP, gaming, video) although these should be taken with a pinch of salt as it's down to what is expected that port/protocol to be used for, so it could be that if your router has a page that displays that.
I wouldn't usually expect your router to keep an extensive history of your DNS requests (however your ISP may do so, so bear that in mind if you plan to visit illegal websites). They would not know what page you visited though (assuming you are connected using HTTPS).
If they had a reason to need to look at your computer they might have seen your browsing history.
Your router may have logs which they can check to look for issues, so could be that.
I've seen some routers where it displays graphs breaking down the types of usage (Web. FTP, gaming, video) although these should be taken with a pinch of salt as it's down to what is expected that port/protocol to be used for, so it could be that if your router has a page that displays that.
I wouldn't usually expect your router to keep an extensive history of your DNS requests (however your ISP may do so, so bear that in mind if you plan to visit illegal websites). They would not know what page you visited though (assuming you are connected using HTTPS).
If they had a reason to need to look at your computer they might have seen your browsing history.