I was in your situation back in 2004 when I needed more bandwidth than the isp could provide with a single connection. I looked into business-class offerings but they were just more expensive without being faster.
My solution was a multi-wan router with 3x cable internet connections. They brought 3x modems, ran 3x coax wires and I had literally 3x full speed-connections. Now with dsl, this may not be possible without them having to run more phone lines since dsl is essentially piggybacking off of phone lines. But if they can, then you can get 3x connections like I had.
The limitations with how you can use these connections together will be the following:
- A single upload/download stream will not exceed your isp bandwidth
- You may or may not be able to assign devices to different wan connections
- You may need to make special entries for certain web sites/connections that expect a single IP per session
These were all I could think of off the top of my head. Now here's what you will be able to do:
- Web browsing will be Nx faster (N=% of isp connections) because a web page is made up of up to hundreds of different elements which will now be retrieved by more than one isp connection simultaneously.
- Uploads can be faster if you are uploading multiple streams since multiple streams will use the multiple isp connections.
- You will have multiple public IP addresses, each of which can be assigned to a different server if you are running servers.
- With a good router, you won't even notice that a wan connection is down because any failover/balancing is seamless.
- With a router that allows this type of configuration, you will be able to assign certain types of traffic to go through a particular wan.
Now, besides just getting the isp accounts, the router is critical to make this work the way you want. Peplink is the expert in multi-wan, but they were always too expensive for me. They can actually mux all the isp connections into one that go beyond what any other router can do, removing the limitations of individual session speeds.
I was using a Cisco rv016 back in the day for my multi-wan setup. It can take up to 7 wan connections, but it's routing capability was limited to 40Mbps--still very viable for your setup. It was discontinued many years ago, but still can be found quite cheap used. This would be a good one to get to test a setup, but because of being eol, I don't know if you'd want to use it in permanently. Cisco still does make newer units in the rv series so that's an option if you like the Cisco way of doing things.
I have also used an older Netgear unit fvs124g. This unit was limited to only 11Mbps in routing so I only used it where I needed less bandwidth, but it did work. It's interface and limitations was different than the rv016 in terms of assigning traffic to wans and vlan capability, and some of that was useful for that installation. I do not believe netgear makes any more pro-series routers. The srx5308 was their last good multi-wan router as it could route full gigabit, but again this is discontinued.
I have also used routers by watchguard. These are enterprise level routers that are more infinitely configurable, but this can also make them daunting to initially set up. Nevertheless, once I had it set up, the reliability was second to none with years between reboots and consistent, seamless performance. I've used the xtm22w, M200, and M300 and while the xtm22w was limited to only 35Mbps wan to lan, it was reliable and was the gateway unit that led us to using the brand.
Prior to buying the watchguard, I was reading the 1000+ page manual for Zyxel's USG series routers that could do multi-wan. I never pulled the trigger on these, but they seemed like they would be reliable once set up. They were also not as expensive as full-blown enterprise routers like the watchguard.
And recently I acquired an old Fortigate 60CM to see how it handles multi-wan. This particular unit is older and limited to just dual wan, but I wanted to get a feel for the interface and how complicated it is to configure. I think this is on par with how the Zyxel would have been with what I have messed with so far. Their newest versions 60F are priced reasonably and almost cheap if you find them open box new.
Sonicwall, Netgate, pfsense and others also support multi-wan as the demand and use of multi-wan in businesses is pretty much essential these days. I don't have any personal experience with them, but believe they are also reliable choices if you like their configuration methods instead.
If I was in your shoes I would first see what you can get from your isp and from other isps including cellular. Then if it does seem like you will have multiple isps, get either the xtm22w or the rv016 and see if you can set them up the way you want them to work. If so, then get the modern version of them and see if that works for you (both have evolved a lot), and if not you know enough about it to know what to look for.
Feel free to ask questions. I've been using dual and multiple isp Internet setups for over a decade now so I've got a little bit of experience.