After many hours of testing i found out that the GLinet router does not “like” my internal wifi card from my miniPC. If i ping my Wifi-Modem from my miniPC. I get ping times of constant 3ms. If i ping the GLinet router. I get numbers all over the place from 1ms to 150ms. (Both these test where made with only one single device connected)
When i add a external USB wifi dongle to my miniPC. Then i also get constant 1ms to the Glinet IF no other devices are connected.
Some results in different scenarios. Pinging from mini PC (USB wifi dongle) to Glinet router:
Only the miniPC is connected ( with new USB wifi dongle) to Glinet router without any traffic or load:

After a second device (My Android phone) is connected it gets worse immediately (still no traffic):

Now i open a website on my android device:

In the syslog i only see DHCP requests and nothing suspicious. The average load of the router is around 4.
Do you know how much fun it is to open websites with a ping time of 1 second? Zero fun. I don’t know what else to do. What can i test more?
In contrast. When i connect my PC with the wifi-modem directly (without the MT300 in between). The ping times basically don’t go up. Even with two devices connected. And that’s how it should look like. Why this doesn’t work with my MT300?: