Engarde

Engarde (by porech): A go network utility to create a reliable IP tunnel over multiple connections.
Duplicate traffic over multiple internet connections using Wireguard.
A self-hosting alternative to Speedify's Redundant mode, without bufferbloat.
Offers ~1ms lossless failover duration, and low latency over lossy connections.
No aggregation, speed of the fastest WAN is the result bandwidth throughput.
No data consumption reduction on slower WANs, since there is no quality monitoring, hence the low-latency feature compared to Speedify.
Processing intensive, requiring 3.0Ghz Intel/AMD router and server for >100Mbit speeds.

Server setup using Vultr (Amazon EC2) as an example with cloud-init: Visit https://my.vultr.com and login/signup, then follow the arrows:
Choose Cloud Compute
Choose High Frequency (since Engarde is single threaded)
Choose latest Ubuntu and the nearest location, example selects European region
Choose a plan according to bandwidth to be used, not hardware resources
Copy and paste the content below then locate and change the password in the pasted content:

Wait few minutes till the IP address appears, and note it down
You can wait few minutes or monitor progress by clicking View Console

Copy the noted IP address & password in the configuration tab of this page and check Enable, Save & Apply
Engarde will automatically detect connectivty via any interface in Active Interfaces section and use it!
All done! Note that the example shows only one WAN plugged in.
Check your IP address at ping.eu,speedtest.net,etc... to check if it's working correctly, it should match the IP address of your server.

Notes:

The server is setup to forward ports from 1024 to 65000 to your SmoothWAN appliance/router, adjust as needed.
Modern implementation UPnP is automatically configured and enabled for automatic port forwarding.
You can also forward ports in VPN -> VPN Port Forwarding.
Changing password is done by simply changing the cloud-init field and doing a server reinstall.
VPN via server providers is (usually) not log-free as it's used for abuse monitoring, privacy may be equivalent to a direct ISP connection.
You can use other providers, Vultr is used as it's the easiest, there is no affiliation.
You may need to modify the cloud-init script for some providers, it's limited to KVM/Virt machines only.
SSH on the server is disabled by default.