Not really many "cheap" options.
Could purchase a netgear DC112A dock, prices range 60-300 dollars e. G. Second hand to new.
You could look into a wifi to ethernet bridge, many of those out there (used to plug in game consoles etc when they have no wifi on the device)
This would be your cheapest option.
What is your budget for this project?
Like @Paddylee there are few options I brought the dock used for $80 with the modem. Wireless bridge around the same price
https://www.umart.com.au/TP-Link-RE450-AC1750-Wi-Fi-Range-Extender_39183G.html . If the modem is always at home then you can get a fix 4g modem like the https://www.tp-link.com/au/home-networking/3g-4g-router/archer-mr400/?utm_medium=select-local , got one used for around $70.