Adjusting The MTU (Maximum Transmit Unit) setting on a router sets the maximum packet size for WAN communication. This MTU setting can be adjusted each WAN interface.
Larger MTU settings provides greater efficiency in data transmission since each packet carries more data. However, if data packets are too large, they may become fragmented resulting in lower transmission speeds or unpredictable results. Optimizing the MTU value on the router’s WAN interface will help to improve communication performance and avoid such issues.
With VPN tunnels, a mismatch of the MTU sizes between the transmitter and receiver can result in problems being encountered.
Another term used is MSS (Maximum Segment Size) which is also used to indicate the largest size of data that TCP will send in a single IP packet. A TCP/IP header is 40 bytes, and the MSS should be at least 40 bytes lower than the peers’ MTUs.

An example of one type of problem that can occur due to a mismatch of MSS is described below. Using a telnet command is used to reduce the MSS to solve a transmission issue.

In this example the customer experienced very slow VPN connections when connection to the router over the NBN HFC connection but had good speeds when using the LTE connection on the Vigor2927L router. At the same time RDP connections had no speed issues. This gives a clue that it is MTU related since the LTE connection used a much larger MTU size than the HFC connection.

The solution found was to reduce the VPN MTU size to 1200 for PPTP.
Telnet to the router and run the following command:

vpn mss set 1 1200

The detailed explanation of the telnet command is as below
vpn mss set <connection type> <TCP maximum segment size range>

Connection Types: 1->PPTP; 2->L2TP; 3->IPsec; 4->L2TP over IPsec; 5->GRE over IPsec; 6->SSL Tunnel

TCP maximum segment size range
PPTP : 1 ~ 1412
L2TP : 1 ~ 1408
IPsec : 1 ~ 1381
L2TP over IPsec : 1 ~ 1361
GRE over IPsec : 1 ~ 1365
SSL Tunnel : 1 ~ 1360

To check you can issue this command:

vpn mss show