Many of these ‘free’ routers are often ‘free to use’ not ‘free to keep’. I found this out when I cancelled my fixed LTE contract with Telkom. Luckily I’d given away my router to a friend, but I had to go fetch it, or else pay the ‘inflated’ cost back to Telkom.
It’s not always the case though with other ISPs, but it is best to check first, and especially so if you cancel a contract before its mandatory period has expired.
This is also why you want to be careful of ISPs who want to tie you into a 2-year contract. You pay penalties if you cancel before that period has passed.
See https://mybroadband.co.za/news/hardware/522765-beware-selling-your-unused-rain-5g-router.html