Have you updated your phone to Android 10 stock before flashing LOS? LOS17 is Android 10 and needs probably needs appropriate updated modem drivers for both slots to work. The Android 9 drivers might not be appropriate.I still have troubles with dual sim - only the first slot is accepted. Both slots work fine on stock ROM.
Anybody can confirm? Or am I missing something?
(start with stock, boot twrp, sideload LOS, reboot system)