How to tie l2tp to the router?

Question

network hardware

Greetings, it appeared that I needed to tie l2tp to the router (due to the connection to the beeline), but, I did not find this item in the web interface. Then I decided to reflash the router to another firmware from this post:
https://habr.com/en/post/444936/
This method helped me and I untied the router from home ru, but I still can’t select l2tp,
That’s what is in list:
1)DHCP
2)static IP
3)PPPoE
4)Bridge I saw that there is a firmware for this router for Russia in which there is l2tp
support, but I could not find it:
center/reviews/2926/
Then I found out that I can’t install another firmware via the web interface, it just doesn’t allow me to do it, but I have a uart programmer on Usb, but I don’t know how to flash this router.
Help if you can, that's my big question.
ROUTER - ZTE ZXHN H118N