USB network adapter or wifi on the newer ones?
Tagged VLANs into a switch? But if the OP knew what tagged VLANs were, then they probably wouldn't be asking questions here
I'll try to make one thing clear. Some of the advice offered previously has been about routers with dual-WAN ports. These are easy to deploy. However what happens with these is that any particular session (e.g. TCP session to fetch a web page) goes over one or other link - both inbound and outbound traffic for that session. This may be useful if there are several people in a household who are using the Internet simultaneously, and to balance their usage between them.
However this *can't* be used to combine bandwidth for a single session (e.g. a single large download), nor to use one link for upload and the other link for download as part of the same session. I think this is why the OP wants some sort of VPN solution, where two VPN connections are connected to some remote VPN node, and traffic is load-balanced between them.
Unfortunately, I don't think this is going to work well.
From my experience, probably the thing most likely to work would be two L2TP sessions with PPP-multilink across them, since this is the same technology used for "bonding" discrete PPP sessions. You need an L2TP termination point at the other end. AAISP offer an L2TP service for £10 per month, but I don't know if it supports multilink. Plus, you need a router which would do L2TP and multilink (possibly their "firebrick" device can do this), and you need to understand how to configure L2TP and multilink.
Maybe some VPN supplier offers a packaged solution for bonding that's easy to deploy. In which case, they'll also advise on what device or software is needed at the client side, to work with their VPN.
But to be honest, whatever you do, I don't think it will work well. You can't really combine two bad links to make one good link. You *can* combine two similar good links to make one good link with twice the throughput. FTTC and 4G have very different characteristics; if you divide your traffic between FTTC and 4G and find performance problems, it will be pretty much impossible to debug. TCP in particular can get very unhappy when packets arrive out-of-order, which will be the case when dividing packets between two links with very different latencies.
I think in this situation, multi-WAN with FTTC as primary and 4G as failover (or FTTC primary for some users and 4G primary for others) is simpler and easier to troubleshoot.