Can you put the smarty mobile sim cards into a 4g router without any issues? If so can anyone recommend a budget 4g router please?

Trying to sort my parents out with a Internet connection.

Just wondering if there are any terms and conditions disallowing the sim to be put into a 4g router?

If not what would be a good small 4g router without spending a fortune? (don't care if I have to go into used market)

It will only be used for one tablet , a mobile phone and a Firestick so doesn't need to be anything fancy.