Info on this is all over the place online and generally unspecific.

Basics:
I have two connections coming from my ISP, going into this switch, which is combining that bandwidth into a single "aggregated" connection.

TP-Link will suggest you download the Omada Controller software, but it doesn't have the options necessary to aggregate the connections. So use the browser access in a moment.

• Use tape to label your modems and lines.
• Get your PPPoE, L2TP, or PPTP username and password for each connection from your ISP.
• (This next part is easiest if you only connect your ethernet cord to one device at a time):
• Login to each modem (192.168.0.1 in your browser) and put them each into bridging mode.
• Login to the TP-LINK ER605 switch (also 192.168.0.1 when there's no other connection).
• Network>WAN>WAN Mode tab; Click however many incoming connections you have.
• Click save.
• Click each corresponding WAN tab above that same screen (WAN, WAN/LAN1, etc).
• Drop down "Connection Type" and select your connection type (PPPoE, L2TP, PPTP).
• Enter the username and password you received from your ISP. (click save)
• Do this for each WAN connection. (click save)

THEN

• Click the Transmission section on the left.
• Click Load Balancing.
• Make sure "Enable Load Balancing" is the ONLY box checked. Uncheck "Enable Application Optimized Routing." (click save)

Should work now.