L2TP VPN connected but not get internet

I establish a connection to vpn using xl2tpd and l2tp protocol in luci

But it can't be established after vpn connection an internet connection
After several attempts, a full connection may be established and Internet access available
But with the same server and pptp connection there is no problem and the connection is established
I tested this with other modems, such as the huawei b612 modem and the connection is easily established with l2tp
How can I fix this?