Hello.
RUT240, firmware: R_00.01.11.2, client connection via L2TP/IPsec with pre-shared key, and id secret: %any. Try 3G and 4G mode (no difference)
Encryption settings: AES128, SHA1, MODP1024
Mikrotik RB750Gr3 - Server L2TP/IPsec: default security settings: AES128, SHA1, MODP1024
Problem: teltonika first connection set encryption is MPPE128, after 30-40sec, reconnect and connect with AES128, then again reconnect after 50sec with AES128 and can reconnect again and after 3 or 4 time of reconnect works normal.
Please make a test with connection to mikrotik L2TP/IPsec server and investigate this problem
With RUT230 client L2TP/IPsec is no problem at all, connection established with first try and work stable.