I'm just an end-user customer, so you should probably check with your Juniper partner for a full view of the potential roadmap for this type of feature.
But I think based on the time past and the lack of announcements of new partners and the EOL on the CX111 that Juniper is not planning on filling this niche. I see two options using other vendors for the wireless gateway.
The CX111 was a rebranded Cradlepoint CBA-750 which is EOL and why the CX111 is EOL but replaced by a newer series of devices.
https://cradlepoint.com/content/cor-routers
The 3G USB modems that were supported on the SRX were all Sierra based and they offer some wireless gateways as a standalone device now.
https://www.sierrawireless.com/products-and-solutions/routers-gateways/es450/