KaiOS powers feature phones such as the Nokia 8110. MediaTek has prepared new 3G and 4G SoCs for this platform. One of these, the MT6731, can also support 4G in dual SIM slots. They are intended to bring new, affordable phones with these connectivity options in 2019
https://www.notebookcheck.net/MediaTek-announces-new-chipsets-for-3G-and-4G-KaiOS-phones.410630.0.html