Here is TP-Links official reply to the situation https://www.tp-link.com/us/faq-2212.html

And which version router are they concerned about on the TL-R600VPN ?I have version 2 and there has been no firmware updates since 2014

Model : Hardware Version : Firmware Version : ISP : What is the maximum downstream signal level the TC-7610 can take? I am averaging around 18.6 dBmV but if I put any kind of attenuation in the line,...

Model : Hardware Version : Firmware Version : ISP : I have your TC-7610 and was wondering what the maximum downstream signal it can handle. Currently I am feeding it about 18.7 dBmV at the highest....