Hi Ahmed,
I'm unsure what you are trying to say here?
/31 subnets - RFC3021 are valid and utilised for point to point links (I use them heavily in a service provider network), but, not all equipment understands it... typically you have problems with Windows clients, but, Linux boxes/standard network stacks do.
https://datatracker.ietf.org/doc/html/rfc3021
That being said, I only said that as a suggestion to something that "may" work here, the fact is that this still feels like a bug in the Teltonika firmware that it is automatically configuring a non-valid /29 link. This would typically work fine with CGN IPs in most situations, but, if you have a provider forwarding a real IP, the /29 boundaries can get in the way and cause issues as I'm seeing now.
e.g. if you are given an IP of 1.1.1.119 from your service provider, this may be done through PPP and a /32 link - this works 100% fine in NAT mode.
If you then select passthrough, the router will disable various sections on Lan configuration (so, I can't really make the change you suggested), and, it will automatically create a DHCP lease for the MAC you put in the passthrough configuration - BUT, you are unable to modify any of the details of the DHCP lease.
So, in the example above, it will automatically create the passthrough DHCP as 1.1.1.119/29 with a gateway (and guessing a virtual IP for the router) of 1.1.1.118/29
The attached device will see the DHCP request and will either fail as .119 is the broadcast address, or, it will apply but it just doesn't work as it should.
In my mind, this entire process just seems buggy and needs a rethink. to me, /31 makes most logical sense and you should always use the alternate for gateway:
So, if the mobile provider assigns .119, you would have a gateway of .118... If the provider assigns .118, you would use .119
Even with this method, there are problems (e.g. if you have 256 sims for a multi SD-WAN deployment with sequential IPs, some may not be able to talk to each other - unless you relay the gateway to the real address?), but the impact seems a lot less as to what it is now.
For now though, again - I do believe this is a bug as if you take my example of 1.1.1.119 - passthrough doesn't work and I have no way to reconfigure it...
