Sporadic IPSec L2TP remote access VPN failures for some users

I have set up an IPSec L2TP remote access VPN server on an Ubiquiti edge router. Clients connect using the native MacOS client.

Some users have problems. Their VPN occasionally disconnects. When they try to reconnect, the connection often fails over multiple attempts. At some time later, they are able to connect again.

Other users have no problems. Their session only disconnect when there is no activity (idle timeout).

The users that have failures tend to be using one specific Internet service provider, but it is not 100% consistent.

The connection failures look as if both sides think that the connection was lost. Thus I am thinking that it is a network issue somewhere between the client and server.

Is there any way to make IPSec L2TP more reliable / network error tolerant?

I am basically limited to config via the Ubiquiti CLI and the MacOS VPN client. But if all else fails, I could edit the underlying config files.

The router is configured based on this tutorial from Ubiquiti. The MacOS client config is based on this tutorial.