I did some extra testing, adding an ethernet switch between the relay unit and the router. Same thing.

So, I guess it is a software issue. The relay unit only supports static IP. I have set it to 192.168.1.15, while the router is at 192.168.1.1. All other devices have leased IPs in the range above 192.168.1.100. Is there any chance this issue has to do with this? I never had any problems before with any router, having static IP devices on the local network. Is there some kind of an option that could interfere with my setup?

@Sunshine Thanx for the reply, good to see there's some support on my issue.

Regarding your requests, here's the version information from the bottom of the MR6400's web interface, please let me know if there's anything fishy:

• Firmware Version:‪1.0.0 0.9.1 v0001.0 Build 210115 Rel.62618n
• Hardware Version:‪TL-MR6400 v5 00000001

Also attaching the MR6400's back side label:

The MR6400 is using the defeult LAN IP address of 192.168.1.1. There are the following devices connected, through address reservation (after they were initially given an IP through the MR6400's DHCP server):

1. a CCTV system at 192.168.1.100, which has incoming port bindings at 8990, 8991 and 8992
2. a security system at 192.168.1.101

However, the issue also happens even when NO other device is connected on the router, apart from the ABB relay unit.

The relay unit has been given a static IP of 192.168.1.15 (outside of the DCHP server's pool), which I suppose would be OK. But it's not.

I will be testing the following in the next couple of days:

1. Changing the relay unit's IP address to something included in the DHCP pool (192.168.1.100 to 192.168.1.199).
2. Disabling DHCP server completely on the MR6400.
3. Connecting the field laptop onto the MR6400, with static LAN IP set up and see what happens.

I will report on the results after the aforementioned tests.

Today I tried a few new tricks:

• Plugged the cable that comes from the relay unit into the laptop, to see if the laptop will get an IP address. It did not, so there's not a DHCP server running on the relay unit, that could possibly be the culprit.
• Changed the relay unit's IP address to 192.168.1.110, so it falls into the DHCP pool's range.
• Disabled the DHCP server on the MR6400 router.

None of the above made any difference. Still, a few seconds after the relay unit's ethernet cable plugs into the router, the MR6400 restarts and keeps restarting as soon as every boot sequence is completed.

Only thing I couldn't try out today was to setup my laptop on the same static IP address as the relay unit, and plug it into the MR6400 and see what will happen. That's a trivial task, however not that easy if your laptop is running win11; it just wouldn't accept the settings, for no apparent reason. I will be trying that out tomorrow, using an older win7 laptop.

Any more ideas are welcome.

I yesterday tried the last trick: Setting the laptop to the same IP address as the relay unit and connecting it to the router. As expected, the router did not restart, and I had normal connectivity to both the router and to the internet.

So it looks like the issue is only relevant to whoever wants to connect the specific ABB relay unit (I tried to link the URL of the unit, but I got an alert that I am trying to post an "illegal extrenal link", what is this? Search for "ABB REF615 IEC" for the unit. I have already contacted ABB support, but the claim that as long as the field laptops behave as expected when connected to the relay units, they've done their part.

I will probably have to switch to some other 4G router manufacturer if this issue doesn't get resolved in a meaningful timeframe.