@lottiemb 

How do you connect these devices together? As a 4G router, the TL-MR6400 need work with a sim card to get internet or a modem when it works in wireless router mode. 

You mentioned that there is no mobile phone signal, what I doubt is that the MR6400 won't work cause it cannot detect the 4G singal from the ISP. 

You can provide more information and then we can give you a specific answer. 

Good day. 

@lottiemb 

Thanks for your explanation. So the TL-MR6400 works fine but you cannot make wifi calling; while as my knowledge, wifi calling makes you call out or text message out with smart phone connected to the wifi, there is no particular requirement for Wi-Fi from wireless router and TL-MR6400 won't block wifi calling packets; 

Below are some questions and answers:

1. What does the HS box do? How do  you connect it to the MR6400? Via ethernet cable?

2. Does ISP support wifi calling service? Have you ever applied for it?

3. Ensure that the IPsec VPN passthrough has been enabled under advanced-Nat forwarding-ALG page. 

4. Try to contact the tech support of HS box or Three to get some help.

Have a nice day.