Internet Drop Outs after Fortinet Install

Without further details about the fgt model/firmware/ISP (WAN) connection, brief network layout, I going to say check the duplex/speed on the WAN connect to the ISP's gateway/modem device.  eg. from the CLI enter diagnose hardware deviceinfo nic <WAN interface)

The output should look similar to this:

# diagnose hardware deviceinfo nic wan1 Description         Intel(R) Gigabit Ethernet Network Driver Driver_Name         igb Driver_Version      5.0.6 PCI_Vendor          8086 PCI_Device_ID       1533 PCI_Subsystem_Vendor          ffff PCI_Revision_ID     0003 PCI_Bus             5 PCI_Slot            0 MAC_Type            6 PCI_Bus_Type        PCI-E PCI_Bus_Speed       2.5Gb/s PCI_Bus_Width       Width x1 IRQ                 16 System_Device_Name  wan1 Current_HWaddr      90:6c:ac:3e:dx:xx Permanent_HWaddr    90:6c:ac:3e:dy:yy Link                up Speed               1000 Duplex              full FlowControl         current:0/requested:3 Interrupt mode      MSI-X Rx queue(s)         1 Tx queue(s)         1 Rx_Packets          42064231 Tx_Packets          32484440 Rx_Bytes            50613981859 Tx_Bytes            4808163593 Rx_Errors           0 Tx_Errors           0 Rx_Dropped          0 Tx_Dropped          0 Multicast           445470 Collisions          0 Rx_Length_Errors    0 Rx_Over_Errors      0 Rx_CRC_Errors       0 Rx_Frame_Errors     0 Rx_FIFO_Errors      0 Rx_Missed_Errors    0 Tx_Aborted_Errors   0 Tx_Carrier_Errors   0 Tx_FIFO_Errors      0 Tx_Heartbeat_Errors 0 Tx_Window_Errors    0 Tx_Single_Collision_Frames    0 Tx_Multiple_Collision_Frames  0 Tx_Deferred         0 Rx_Frame_Too_Longs  0 Rx_Frame_Too_Shorts 0 Rx_Align_Errors     0 Rx_Flow_Control_XON 0 Rx_Flow_Control_XOFF          0 Tx_Flow_Control_XON 0 Tx_Flow_Control_XOFF          0 Rx_Control_Unknown_Opcodes    0 PHY_Media_Type      1 max_frame_size      1522 CTRL                00100241 STATUS              00280783 TXCW                00000000 RXCW                00000000 avd/ctrl            0de1/0200 Check the output for (RX/TX and other) errors.  If there are errors/drops, wait a awhile then perform the diag test again to see if the error counters increase.

If you need to set/force the speed/duplex on an interface, use:

config system interface edit "<WAN interface name>" set speed <value> next end Where speed value is auto        Automatically adjust speed. 10full      10M full-duplex. 10half      10M half-duplex. 100full     100M full-duplex. 100half     100M half-duplex. 1000full    1000M full-duplex.