It sounds like the game is preventing you to join the server because your ping is exceedingly high, for example "600ms" or more. The game stops people joining the servers with high ping to allow people to drive on the roads without someone pinging backwards and forwards and doing 99% damage to someone else's truck unintentionally. Try and contact your Service Provider and ask them if they can diagnose the problem, maybe get a engineer sent over to replace your router, or place the router in a better location. Good luck!
**edit** " Speedtest: 7ms ping, 71Mbps download, 60Mbps upload " Remember that this is a speed test that is connected to a server that could be closer to you, and with less people using it. I would recommend using PingPlotter to identify the problem. PingPlotter can show you if the high ping is coming from the game server, or your internet router. If you are interested in PingPlotter then go to the site https://www.pingplotter.com/ and download the app.