I'd agree with what candlerb says.
I also have hilariously unreliable FTTC that Openreach think is 'good enough' and reliable 4/5G but the former has super low latency and the latter is fast but high latency.
If you want bonding, you need to use a service to 'combine' the streams on the other end. The only ones that do that are essentially 'SD-WAN' providers, which are proprietary and £££ (Cisco Meraki, Silverpeak, Peplink Speedfusion, VMWare Velocloud etc). The reason for this is that SD-WAN is non-standard and still 'new', but essentially all involve putting a VPN to a concentrator and then managing the network via various technologies including OSPF and link aggregation and so forth. The point is you can use any ISP or connection underneath them for the 'physical network', but your locked into the tech.
Opensource SD-WAN exists, but it's very 'beta'. OpenMPTCPRouter is a modified OpenWRT to do MPTCP, but its a little unnecessarily complicated and really feels more like a testing ground for using multipath than a solid solution. Another is FlexiWAN, which allows 3 free devices. But both of these are a bit ropey. If you want an open source it's best just to have a VPN to your own VPS or server and just use simple aggregation across these links, but it's fragile.
Finally you have ISPs. Cerberus
all provide bonded broadband solutions as a fully managed package. Prices are more than a multiple of X lines due to the need to use more expensive CPE equipment.
Finally the other option to bonding is failover. 'Failover' between 4G and FTTC and back again is considerably easier. In fact most cheap routers will support failover which just involves pinging a host continuously and flipping the link to keep up time, but you're only using the bandwidth of one link at a time.