I purchased and installed a TL-MR6500v 4G LTE telephone wi-fi router at my elderly in-laws house as they have never had broadband and I needed to be able to install a Ring Doorbell and monitor it remotely.

This router (with an ID mobile SIM card) does this perfectly allowing access to the doorbell video.

I was also hoping it would also support a digital house phone, but while this works 90% of the time, I'm having issues and need some help from TP-Link Support.

The router has been installed for a couple of months now and in that time the phone that is connected to it has stopped being accessible on three occassions.  By this I mean, when dialling the number of the SIM card in the router, a recorded message indicates the number is not available.  Once this happens, the phone is useless and it is necessary to drive to my in-laws to sort out the issue.

I've found that unplugging the phone from the router, rebooting the router and power cycling the router make no difference; I still get the recorded message when calling the SIM card.  The ONLY thing I can do to fix the issue is to make a call from the phone connected to the router.  Once this call has been made, dialling the SIM card works again i.e. there is no recorded message, and the phone rings normally.

So, it would appear that once the phone "breaks", the router doesn't recognise that the phone is connected to it, even when powering off etc.

I have installed the TP Link Tether app on my android mobile phone in the hope that I might have been able to "fix" the issue remotely, but I can't even get the app to connect to the router, unless I am within wifi range as it says "Cloud Device (Offline).  I did try using the app to reboot the router while at my inlaws and connected to the routers wifi, but like all other attempts to fix the phone issue, this didn't work.

Any thoughts on how to get to the bottom of this?  As it stands, it looks like we will have to revert back to using the BT landline, which will be a real shame.