We ended up going with PepLink's SpeedFusion to save engineering time. But the license was costly. I really hope for a free solution in the future for 2 cellular networks and <50ms failover.
Multipath UDP + OpenVPN would also probably be a viable solution.
It will connect your devices in a P2P Mesh VPN and allow them to send and receive data using multiple links (e.g. multiple 5G or 5G + Satellite).
It is significantly cheaper than Peplink's license, less latency and no bandwidth / data limits.
You need to bring your own hardware though. Like a Raspberry Pi with 3 USB 4G/5G dongles.
https://mikrotik.com/product/rbm33g#fndtn-specifications
maybe someone could make one that uses an RPi compute module instead.
I found this board on AliExpress (https://www.aliexpress.com/item/1005003540616473.html?spm=a2...) based on the CM4 and with 3 cellular modems. That could also be a good candidate.
Also found this one (https://www.gateworks.com/products/industrial-single-board-c...) US-made and has 3 minipcie slots (other options available with 2 and 4)