First some background. We have flow monitors installed in sewer manholes. They monitor sewage flow. They collect the data and send it wirelessly to a server so we can check the data without having to physically go to the manholes. The modems are integrated into the meters. They are 3G modems and 3G is not going to work for much longer. Is there a device/method that can pick up the signal from a 3G modem and translate it to a 4G network?
I wouldn't think so because 3G is a connection to a 3G network, like a cell tower, and if the network provider and hardware no longer supports 3G, then that's the end. This is probably a custom hardware need and you would need to approach a company like CISCO, Netgear, etc. to create hardware for this. Are there special hardware companies that cater to your industry? You might need to make an (expensive) deal with them. Where did your company's original hardware system come from? Maybe contact them for an upgrade?
https://www.google.com/search?q=3g+to+4g+converter https://www.invertersupply.com/index.php?main_page=product_info&products_id=199276