Mmmh, maybe the issue is that you need autonegotiation (say if you have a straight instead of a cross-over cable, but you also need to restrict the speed)
ethtool eth1 -s advertise 0x008
From man ethtool:
-s --change
Allows changing some or all settings of the specified network device. All following options only apply if -s was
specified.
speed N
Set speed in Mb/s. ethtool with just the device name as an argument will show you the supported device
speeds.
duplex half|full
Sets full or half duplex mode.
port tp|aui|bnc|mii
Selects device port.
mdix auto|on|off
Selects MDI-X mode for port. May be used to override the automatic detection feature of most adapters. An ar‐
gument of auto means automatic detection of MDI status, on forces MDI-X (crossover) mode, while off means MDI
(straight through) mode. The driver should guarantee that this command takes effect immediately, and if nec‐
essary may reset the link to cause the change to take effect.
autoneg on|off
Specifies whether autonegotiation should be enabled. Autonegotiation is enabled by default, but in some net‐
work devices may have trouble with it, so you can disable it if really necessary.
advertise N
Sets the speed and duplex advertised by autonegotiation. The argument is a hexadecimal value using one or a
combination of the following values:
0x001 10baseT Half
0x002 10baseT Full
0x004 100baseT Half
0x008 100baseT Full
0x010 1000baseT Half (not supported by IEEE standards)
0x020 1000baseT Full
0x20000 1000baseKX Full
0x8000 2500baseX Full (not supported by IEEE standards)
0x1000 10000baseT Full
0x40000 10000baseKX4 Full
0x80000 10000baseKR Full
0x200000 20000baseMLD2 Full (not supported by IEEE standards)
0x400000 20000baseKR2 Full (not supported by IEEE standards)
0x800000 40000baseKR4 Full
0x1000000 40000baseCR4 Full
0x2000000 40000baseSR4 Full
0x4000000 40000baseLR4 Full
Maybe that does the trick, assuming that eth1 is your wan interface 
Please note:
So this is not 100Mbps, but still gigabit ethernet, I guess I gave a wrong command it should be:
ethtool -s eth1 speed 100 autoneg off duplex full
But potentially your problem is not so much the autonegotiation of the bandwidth but rather the wrong cable (see https://en.wikipedia.org/wiki/Medium-dependent_interface) at least I see no "MDI-X: on" as part of your ethtool output indicating that the ethernet port might not do this. Under that hypothesis the switch might help as it might do the MDI-X thereby fixing your issue... just a hypothesis though. (Says the man that once spend an hour trying to get a link to operate at 1000Mbps with ethtool and friends before realizing that the 4-wire cable connecting the devices under test simply was lacking the additional 4 wires for gigabit ethernet....)