@Rostagroup 

You will need 4G hotspot/router, and it must have an Ethernet port. See ZTE MF279T as an example of such device. Connect Main Deco to 4G router Ethernet port, and you are all set.

Start with checking what your wireless carrier offers, you may be able to get a discount if you already have cell phone plan with them, and support may be easier to get. As well, there won't be an issue with incompatible devices, if it is from same wireless carrier you have cell phone with - their 4G hotspot/router will definitely work in your area.

This will not be cheap, you will have to pay for device and most likely need to buy data plan for it. Depending where you live, that data plan may be pricey and possibly not unlimited. Also, you'll be underwhelmed by Internet speed from 4G, especially coming from fiber.

Check if your area is covered with 5G. You might get better deal on 5G router and data plan than on 4G, because 5G is relatively new technology and some wireless carriers could subsidize it for new users.

@Rostagroup Maybe you have an old router that you don't use anymore? Then you can create a mobile hotspot with your phone and connect the old router to it wirelessly. Then connect the main Deco to the old router instead of your fiber modem/router.

example: https://www.youtube.com/watch?v=kp-lCLZtwwk

Obviously it will be slow and possibly expensive.