Re: Aggregating 4G
I'm going to assume this is an Android phone, because otherwise you're probably out of luck as the OS is probably not modifiable at a low enough level to allow it. Assuming it is Android and that it is rooted then technically there is likely no reason you can't do this on the Linux level by bonding the network interfaces in some way.
However there may be a number pitfalls to doing do that outweigh any advantages. VOIP, video calls, certain streaming video services among various other things may not work as you'd expect. I foresee one of two possibilities with these and other services:
1. They will work but will only connected via one of the connection paths, so not giving you any benefit.
2. They will work initially but will drop connection, streaming video will buffer extensively and VOIP/Video calls will drop frames or break up.
Most apps and services are not designed to handle having their network traffic presented through two different routes, if the traffic switches between routes it will assuming the traffic disappearing off one route and moving to the other will be seen as a disconnection.
This is unfortunately inherent in the nature of TCP/IP traffic. TCP is a single path protocol and was never intended to support multipath traffic in this way. Multipath TCP (MPTCP) does exist, but has only recently been properly standardised and is not widely implemented.
Link aggregation, and multi-pathing for failover, are of course both possible. But in general they have to be obfuscated from the endpoints allowing them to communicate from a single address each, as they are usually designed to work on a single path network platform.
Doing this in your scenario would require a host server, with sufficient bandwidth to match the two combined connections, two aggregate the links for you and be the outward facing point to the Internet. I've known of ADSL broadband ISPs that offer such a service in the past, but I've never heard of a mobile operator that does it.