Buy a proper dual wan load sharing router.
They are not that expensive now. You can get a simple one using a very ordinary router and 3rd party firmware like dd-wrt. It tends to only use round robin type load sharing .. but that is enough for most people.
Or DIY with a router software like PFsense on an old computer.
Or buy next level SME router.. Mikrotik or Ubiquiti. Draytek have a good reputation although it has become pretty expensive... it is easy to configure cf the more command level devices.
Or go ebay /second hand dealers hunting for ex-leased equipment. You can buy excellent grade stuff really cheap if you are prepared to shop around. I have bought enterprise level routers like Linksys, Netgear and Dlink. There are usually plenty available because business buy new or more likely lease and they are too complex to setup for typical home user. When I used to do computer support we used snapgear and installed heaps of them.