Can I connect it to the (salt glass) router from the provider via rj45 and then only use the wifi from this device and switch off the wifi from the glass router?
You turn off the WiFi on which router, the Salt or the TP-Link?
Can you manage to log on to the TP-Link? Then you have to change the router to Access Point in the top right-hand menu (you can select the language to the left of it) under Mode. Have you ever got this far?