In general your plant does not look too bad. Brown stem is normal as it matures but mushy stem is not good. Lack of light is making it look a bit lanky, especially that one branch to the side indicates it is trying to extend towards light. From the pics it also looks like the soil is very wet. Is that glass thingy a watering device? It is probably keeping the soil wet too long. Ficus would like to be on the dry side and especially in winter when growth is limited. Does the pot have drainage hole or is there a second pot inside the larger black one? If so you will have to also make sure that the bottom of the inner pot does not sit in water and become soggy. If there is no drainage hole at all then it needs to potted into one that has immediately.

First of all - thank you so much for your answer!
I cut the mushy stem one yesteray and placed the healthy leftover stem and leaves into the water - let’s hope they will give me some roots.
Lack of light in old house was a problem and i didn’t rotate it there at all which means the tree was facing for almost a year to one direction that might the reason for lankiness. That is why tree is currently facing to other way (hopefully it will straighten up).
Yes - soil is wet because i did water it in the shower last week.
Glassy thing is kinda device that shows when the tree drinks and when its the next time to water it.
There is second pot inside with drainage.
I still think it might the root rot and not sure if its very wise to repot it winter times.
Here in Finland it’s really cold now and the light time is short.

ALSO i noticed that some of the leaves are turning brown

Your soil looks like it is mainly peat based and it retains a lot of water. And that can easily lead to root rot. My suggestion is to remove that watering device - it is not letting the soil dry out. A much better method is to use a chopstick or a similar wooden stick like dowel rod and insert it in the soil to measure deep in the soil. If it comes out dry then water it well. After watering drain it well. To help with draining you can tilt the pot and/or shake the pot up and down a few times. You will notice a bit of water will drain out. Another method is to let sit on an old dry towel for 10-20 minutes to draw out the excess water. You really need to let the soil dry out a lot.

Scratch those brown spots with with your nails. If they come off then it is likely scale insects. A qtip or tissue paper dipped in alcohol can be used to wipe them out. Look for them on the stem and crevices/corners too. If they do not come off then I would not worry about it. If you have larger patches of brown then it is a different matter.

Consider adding a supplemental light. Even a 100W equivalent LED light will be helpful.

Finland is a beautiful country and always wanted to visit. Light must be really low and weak now. So a bit of extra help for the plant will make the plant happier.

Also consider this post on Ficus by our resident expert Al: Ficus Trees in Containers IV

It is quite long but you read the part by Al himself and get an understanding of how to care for a ficus.

Be careful not to tie that cord to tightly so as not to girdle the trunks.