Slow internet speed through FWF30E

[CCST]

Our customer got an upgrade from their ISP and with that a new modem. Now the internet speedtests show 3 mbps down and 10 up. If we plug a computer directly to the modem, we get 100 mbps down and 10 up.

One strange thing is that if we use the ISP's own speedtest, we get around 60 mbps down and 10 up through the Fortigate. We have spoken to the ISP and they claim the fault is in our equipment. I have checked the wan port and there is no custom MTU settings applied. Speed of wan interface is 1000 mbps and full duplex.

Description Marvell NETA Gigabit Ethernet driver 00000010
System_Device_Name wan
Current_HWaddr 70:4c:a5:22:11:61
Permanent_HWaddr 70:4c:a5:22:11:61
State up
Link up
Speed 1000
Duplex full
Rx_Packets 26878
Tx_Packets 24300
Rx_Bytes 19236130
Tx_Bytes 8121040

if=wan family=00 type=1 index=4 mtu=1500 link=0 master=0
ref=146 state=off start fw_flags=0 flags=up broadcast run allmulti multicast
Qdisc=mq hw_addr=70:4c:a5:22:11:61 broadcast_addr=ff:ff:ff:ff:ff:ff
stat: rxp=28972 txp=26276 rxb=20229781 txb=8594900 rxe=0 txe=0 rxd=0 txd=0 mc=0 collision=0
re: rxl=0 rxo=0 rxc=0 rxf=0 rxfi=0 rxm=0
te: txa=0 txc=0 txfi=0 txh=0 txw=0
misc rxc=0 txc=0
input_type=0 state=3 arp_entry=0 refcnt=146

[CCST]

I am considering involving support, but I'm hesitant to go through all that red tape. We may just replace the FGT with the ISP provided router.

Thank you for your help in this so far.