Review TP-LINK TL-R480T+

It will likely work as good as you can expect for a device in that price range. Bigip would not sell their F5 load balances for $10000 if a $100 box could do similar function.

You need to read and understand the words they use very carefully. What a dual wan device can do is say run 2 medium definition stream from youtube at the same time. It can not combine the wan connection together to get the ability to download a high definition stream. The maximum a single session can ever get is only as fast as a single WAN connection. Load balance does not mean combine it means it spread different session over the different connections.

On top of this issue you still have a massive issue of actually keeping the right sessions on the right connection. This is what a F5 tends to do a better job at. The example I like to use is say you are playing some online game. It has one central login server and then multiple actual game server. If the connection to the login server goes over wan connection 1 and the connection to the game server goes over connection 2 you are now attempting to connect using 2 different IP addresses to what should be a single game instance. The game company will detect this as a hack and disconnect you. So how do you ever get a router that can figure out how every company works. There are some methods to alleviate some variation of this problem which is why bigip can charge so much but even they likely won't have a solution for all the issues.

What I would do to start is just use your PC itself to do this. Turn off the DHCP on one of the router and plug the lan ports together. Set the IP of the secondary router to say 192.168.0.2 ..assuming our main router is 192.168.0.1.

What you can now do is "load balance" the connections yourself. You can use the ROUTE command to send traffic for some IP to the secondary connection. So if you wanted a site with ip address 123.123.123.123 to use the second connection your would do
route add 123.123.123.123 mask 255.255.255.255 192.168.0.2

Playing with this for a while should show you how tough this issue really is. Generally it only works good if for example you wanted your VPN to work to always use 1 connection and you general surfing to use the other.

Still if you are persistent enough you can use both connection at the same time effectively.