Hi:
You'll get several different answers to your question, including some advanced commands to forward or "fork" calls between two OBi products. However, let's start with this:
Please quantify "far away"? How many feet or meters? Have you tried using a DECT 6.0 phone, before ruling it out? I have several DECT 6.0 products, and all of them have excellent range, up to a half-block away (I could take the phone to the next-door neighbor's house and use it, for example). DECT is a digital protocol, and, while it isn't "three letter government agency proof", it is reasonably-well encrypted, so privacy isn't an issue.
Given that you have no equipment yet, but you do have wired Ethernet available at both locations, would you be amenable to having one or more hardwired VoIP phones at those two locations? Obihai makes excellent IP phones (models 1022, 1032 and 1062). You could simply use one at each location, or one at one location, and a OBi 200 Analog Telephone Adapter (ATA) with either a conventional desk phone or DECT cordless phone plugged into the ATA at the other location. No call routing needed; both phones can separately register to a VoIP service provider and make/receive calls independently, and you can call one extension from the other.
I should add, that, while there are WiFi solutions available, it's pointless for your application since a) you have hardwired Ethernet available at each end, and b) DECT 6.0 actually has longer usable range than WiFi.