Dual active LTE + multipath tcp + VPN
Android's multipath TCP networking layer is only able to use WiFi and Cellular concurrently and can't use multiple of the same connections at once (as far as I know).

However. You can use a mobile WiFi Router from Huawei for example. To get a seamless LTE and WiFi over LTE connection. The Huawei router is very fast and as big as my phone so very easy to carry. It offers about 8 hours of coverage but they have models that are slightly bigger that offer 20 hours.

This way you can use for example Samsung's download boost to use LTE and Wifi at the same time to get double the speeds/reliability. I would suggest you get sim cards from 2 major different providers for extra reliability so if one is under maintenance/has low coverage you can rely on the other.

I hope this helped if you have more questions feel free to ask.

*If you want to double your speed you might also want to look into NIC teaming I think this only works on windows / pc's though