Is it safe to purchase a second hand modem on eBay?

I have bought a new router to replace my ISPs router (which is no longer being updated so doubly problematic). Unfortunately despite my research I bought a flint 2 which I have discovered does not work with a FTTC connection and the type of authentication I need for my ISP. Full fibre is not yet available in my area. So I need a modem which does this (I can’t use my isp’s router as a bridge). I have found that these are available for a reasonable cost on eBay but I have no idea whether there are any privacy or security issues with doing this? Could someone download anything to a modem that could be used for nefarious purposes or for tracking or anything? Or any privacy/security issues I haven’t thought of? I’ve been nervous about asking this as I suspect this is a new really stupid question but I know very little and am gradually trying to build my knowledge and understanding which this forum has been invaluable for with its many very knowledgeable people. I have been caught out by my assumptions before doctoring not to make that mistake again! Thank you!