LAG causing loop?

I have R8000P and GS116Ev2. R8000P WAN port connects to the modem and works well (with internet) without the switch.

1st experiment: When I connect R8000P LAN1 to GS116Ev2 LAN1 in the default setting (no LAG), all other ports from GS116Ev2 get internet access. I have also changed my other cables to make sure the cables are working well.

2nd experiment: Set R8000P LAN1 and LAN2 as static LAG. Added GS116Ev2 LAN1 and LAN2 into LAG membership 1, then enabled LAG. Now, R8000P LAN1 connects to GS116Ev2 LAN1, and R8000P LAN2 connects to GS116Ev2 LAN2. My computer connects to R8000P wirelessly and can still access the web management of GS116Ev2. Both LAN1 and LAN2 on GS116Ev2 show status "up," but no other port on GS116Ev2 gets internet access or can access the main router.

3rd experiment: With the same hardware setup as the 2nd experiment, but I enabled loop prevention on GS116Ev2. Now my computer cannot access the web management of GS116Ev2. No ports on GS116Ev2 get internet access or can access the main router.

Any help would be greatly appreciated!