If they went offline while they were there, that might be suspicious, but a day or two later I’m chalking up to coincidence. ISP routers are typically not the greatest, it may just be a lousy model that freezes up a lot. If it is in an area with poor ventilation or high heat, try to get it better airflow or move it somewhere cooler, but other than that, you’d probably get better performance and save money by buying your own modem and router.
If you want to keep theirs, you could put it on a smart plug or even an old school analog timer and have it set to shut off every day at like 3AM and turn on at 3:01 (or for the old school timer, as short of a time as it allows, probably 5 to 15 mins).
This isn’t the great for electronics to have the power cut like that repeatedly, but it will usually take a long time to do damage (the analog timer isn’t as clean so it might affect it more quickly, digital or smart is better) but they own it, so you just get a new one if/when it dies.
To be extra safe, change the at least the password (ideally the username too) to your modem/router’s admin to something other than what is printed on it. They can’t get to it from outside your network, but they could drive by and get on your wifi and mess with it.
Change your wifi password too (again, not something printed on the device) and if you are letting them use your wifi, set up a guest network, those should not be able to access the router admin even if they have the password.
