Connection issues can be the result of a wide variety of causes.
All of our servers are located in the United States, so if you are not in the US, geographical factors could be contributing to high latency. Additionally, many mods can consume a significant amount of bandwidth, which might affect your connection to the server.
For further diagnosis, run a speed test and see if your connection is also experiencing problems. If it is having issues, the next best step is to contact your internet service provider (ISP).
On occasion, the Modrinth Servers configuration may be incorrect for your server. If your internet is stable but you are getting a "Can't connect to server" message, you can try changing your server URL. Changing it to any value then restarting your server should work. After about a day, you should then be able to change it back safely.
If you are absolutely sure there are no contributing factors and you are just experiencing poor connectivity to our servers with no chance of interference, you may contact support using the green bubble.