@Themelster Hi, please go to Advanced > Network > Internet to verify that Data Roaming is enabled on the router web GUI, and ensure also your SIM card does enable roaming. You can test it by...

@rongonsalves I see. May I know when did you purchase it? Does this issue happen recently? Are there any change in the network, such as firmware updating, power outage, etc., anything that you were...

@Gunt Try to login to the router web GUI to confirm if the wireless network name had been changed, then run a Wi-Fi analyzer on a computer to confirm if you can find the corresponding SSID. If no...

@Indra123 Do you still use the original power adapter? Was there any power cut or bad weather when updating the firmware? Try to hard reset it by holding the reset button or hole for 10s. Are you...

@Markinator Can we have some details about the issue "keeps rebooting when switched to 4G"? What is the phenomenon that you can observe when the issue happens. BTW, who is your ISP? Is it possible to...

@Arihant Is the 4G connection is not stable on the MR6400, you may check if this article will help: https://community.tp-link.com/en/home/forum/topic/265474