Good evening,
I have the same problem. I can't find a solution to this problem yet.

  @Sunshine Thank you for taking time out to suggest potential problems and apologies for replying late.

I did went away did some additional test. The primay cause of the problem that I determined is that the LTE provider was not issuing a IPV 4 address. In other words, not a router issue, but a Mobile network issue.

What I did... Rest the Device and started clean. Then,

SIM1:  I was using the SIM of Reliance Jio, India (Provider 1). I figured that it wasn't issuing any IPV 4 address. See picture 1 below.

SIM 2: I used an Airtel SIM, India (Provider 2) to validate if it exhibit similar behaviour. Interestingly, this provider issued a IPV4 address. See Picture 2.

If anyone is not able to access an IPV address directly from their device connected to LTE router, then the culprit is likely your mobile network and not the TP Link product.

I intent to contact the customer care of the provider to check if this is a setting they can enable or we have have to live with IPV6. 

Picture 1 and Picture 2

 @Sunshine I did further investigations and I believe there remains the primary issue of how to client device naviage to a IPV4 only address. There are instances, where the client device is trying to access a IPV4 host only - either by specifying the URL or by specifying the IPV4 address. However, the DNS (Auto provided by Jio Operator) is not resolving this and basically returns nothing. There is perhaps an issue with both Jio Network and this router model.

1. The local LAN only runs on IPV4 mode. While there is a provision to specify a IPV DNS resolver (eg: 8.8.8.8), it doesn't help as the Jio Network do not even issue a IPV4 address to the router. 

2. I wanted to try the Google DNS64 IP as manual DNS setting - however, there isn't any provision to manually provide IPV6 DNS Address as the router is hard coded to run only on IPV4. 

3. I am not a network expert but it appears that DNS64 facilitates communication between IPV6 only clients vs IPV4 only clients. Now, is this a configuration error at the Jio network's end or a hidden router configuration is not something I am able to opine. 

@Sunshine : Wondering, if there is any test that you are able to perform to check where the problem lies and what efforts that TP link is able to lead to engage with the mobile service provider if that is improtant to fix this error. I believe a collaboration effort will help.