Is there a way to turn my phone to landscape mode in the lock screen?
Your device seems to support USB OTG functionality. Given that, you can buy an OTG cable and use a mouse (any mouse would work) to control the device, just like you control a computer using a mouse. 

Once you unlock the device, attempt to root it, then install Xposed Framework for Nougat, followed by installation of GravityBox [N] Xposed module.

In GravityBox module, there is a setting named Enable lockscreen rotation under Lockscreen tweaks. Enable it and restart the device. The rotation control should work on lockscreen now. 





Note: If the device is encrypted, and you reboot the device, the rotation tweak would not work because the apps are not loaded by that time. You would have to use your fingers or the mouse in portrait mode to unlock the device.