I have done umpteen GPS tracker installs for customers and if you don't need an insurance approved one you can operate two trackers on a superb commercial server totally for free, and as many as you want (thousands of you like) on your own server at home or work, or on your phone with other free software.
Be SURE to only buy a 4G tracker, there are ultra cheap 2G ones for sale but gprs data on 2G is closing. EE already had a whoopsie where 2G gprs was accidently shut off prematurely and 2G trackers could only be contacted via text for about a month this year in some areas.
4G will give you some good future proofing. I recommend genuine Teltonika ones. A basic Teltonika is about £60, and with a free server like Ruhavik ongoing costs would just be the data SIM costs, a fiver a month tops.
Finally some cheap trackers are tied in their firmware to a dedicated commercial server that starts off free but then becomes PAYG, and some will only update every minute to the locked in server, to save traffic. You can't put in any other server on these.
Google Traccar for a free if rather cumbersome server software for phones and PC's, and Ruhavik for a free server that allows two devices on their basic package and works flawlessly with an easy to use interface.
Happy to advise further, it's a bit of a minefield.