In place where I'm living now, I have a poor internet connection. My download speed is not bad, but upload is poor. I have two broadband providers. First is DSL, or isdn I'm not sure, but I know one think working on annex b. My second provider is LTE -- its a typical SIM card. Several weeks ago I found this website, and now I'm wondering on one thing: is it possible to connect (to unite) two different technologies in one? For example, connect DSL and LTE in one to have the combined download and upload speed? If this is possible what sort of router and dongle for SIM card would I need?
Multipath TCP (RFC 6824) could be a solution to explore. In my understanding you can split a single session across different technologies, so it should be possible to divide up- and downlink and spread it across different links. That being said, it is an ongoing effort so actually using it requires to employ an experimental Linux kernel module (download and documentation here) and to build a Linux router that operates that way.
The term is bonding. And no, you, the end user, cannot combine the connections from two different ISPs into one larger pipe. You can load balance between the two, but the speed will be limited to the connection selected.
That said, there are ways to get the same net effect using a 3rd party service. Sort of a "multilink VPN", but I wouldn't recommend it.
I agree with Ricky Beam that bonding cannot be accomplished with discimilar WAN technologies from different providers. However, you can do some load balancing with fail-over if your firewall is intelligent enough. Here is a how-to from the pfsense people should give some fairly good insight: Multi-WAN
Having said all of that, it appears your greatest desire is to increase your individual download speeds and this will not address that directly as Multi-WAN operates on a connection-based round robin basis.