Why is it on our Minecraft server?

The three of us bought a Minecraft server (Nitrado). Unfortunately there are inexplicable lags and the support can't help us. The situation is this: my brother and I (we play in the same Wi-Fi) have lags again and again and our friend who plays doesn't have any. We then bought more RAM and our server was moved from support to better hardware. The lags then stopped for a few days, but unfortunately started again sometime. We asked support again and they couldn't help us because there were no CPU or RAM problems.

The strange thing here is that my brother and I only have lags when we play together and when we play separately the lags are minimized. Our friend doesn't notice any of the lags during these times. We checked our ping several times and it is always on a good value, which is why I exclude that it is due to the internet. In addition, other games online namely lag free. I can just move to the server, but when I mine blocks I don't get them until 10-30 seconds later. For example, when I type messages in the chat, they are immediately visible on the console, but not in my own game. We also performed chunk fixes several times, but these didn't work.

I hope for answers.

Thanks for reading for a long time.

Edit: How to play with mods and my brother plays with a LAN cable (I don't)

Measure times when you play together your internet bandwidth with a speed test.

It is not uncommon for a connection via Wi-Fi to have less internet bandwidth than a LAN connection. Wi-Fi is already a bad starting point for a laptop. If you play in the same network for two, then you share the bandwidth. So completely normal. That there can be lags. Even constant time outs can result if more people use the same connection.

There's no sensible solution, except for changing from Wi-Fi to LAN (cable).

My brother plays with a LAN cable and I with the Wi-Fi

Then check your latest.log if there are any errors with mods. These can also cause lag. If everything is fine, then you only have the LAN cable.