Connection issues can be the result of a wide variety of causes.
Modrinth Servers has datacenters in the United States, Europe, and Southeast Asia. If you're not sure which datacenter your server is in, you can open a support request to have it be changed to any of the other datacenters free of charge. Geographical factors are one of the most common factors contributing to high latency, so transferring it to another datacenter can often assist in such cases.
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. Change it to any value, then change it back, then restart your server. If the problem persists, try connecting to the server using its direct IP address and port number to bypass potential DNS issues.
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.