You can connect the TP-Link 4G router to the ASUS router via a LAN cable. The ASUS router supports connection via LAN, but it does not have an integrated 4G modem function. You can use the TP-Link router as a backup Internet connection by connecting it to the ASUS router via a LAN cable. Note, however, that the dual WAN function must be configured via the ASUS router's web GUI.
Hello Adrian 1974
No, you don't have to. I have 3 devices in a mesh network via WiFi. The devices also use a separate channel for this. However, it is important that the devices have a good connection - i.e. the positioning is relevant.