The remote VPN connection to the subnet is falling, what could be the reasons?

Question

VPN

Good day to all who answer my question.
Given:
Server + TP-Link router => VPN Tunnel => Remote ZyXel router from the provider => our TP-LINK router with configured VPN => Remote PC
The network is configured correctly, tested on different types of pairs of routers (TP-Link =>TP -Link, Comtrend => TP-Link, Asus => TP-Link) from time to time the connection to the PC drops, that is, the Internet is present on the PC, there is data transfer from the computer, but it is impossible to connect to the PC via VPN, and from the server side You can see that this connection is inactive. Sometimes restarting both routers helps, sometimes not. The firmware on the routers is fresh.
There are no problems on other pairs of routers.
Question: What can "drop" the connection?

Answer 1

Alternatively, errors in the implementation of VPN on routers, or errors in the operation of VPN traversal on Zuhel. The most likely problems are in the work of TP-Link, which is a VPN server (up to overheating, by the way, which can screw up the math of crypto calculations in its processor and connection setup errors until the overheating disappears).