Based on your explanation (or maybe my assumptions), you would not want to use Mesh in your case. A WDS repeater is mostly likely what you want, although perhaps a WISP repeater would work as well.
Mesh is designed if there will be several wireless hops from some of your clients back to the device providing Internet service. Most homes are not large enough to make this necessary. In larger buildings (e.g. hospitals, factories, schools), it may be too costly to retrofit the building with Ethernet to every corner of the building. In such cases, Mesh is very helpful and cost effective.
GL-inet has provided an easy way to configure a Mesh using the physical buttons on the B1300, as explained in the manual. However, a bunch of changes are made when this is done, not all of which are likely visible from luci. So, further configuration within luci of a Meshed B1300 is likely going to lead to problems for most people. If you have tried to mesh your routers, I advise a full factory reset before proceeding (to change back all the settings, even some that may not be visible from the gui).
With only one wireless hop needed, as it seems in your case (and most home cases), a repeater is much easier and straight forward. WISP is the easiest to configure, as it makes no assumptions about the “main router”. However, WISP will create a separate network “behind” the repeater, i.e. if your main router gives addresses in the space 192.168.20.X, the repeater would give addresses in a different space, e.g. 192.168.111.X. This will make it harder for devices from the two subnets to directly communicate. When you are in a hotel, this may be preferred, as it gives some protection to your devices from those outside your room, but most home cases, it is not preferred.
WDS is likely what you want for your home. Since you have two identical GL-inet routers, WDS should work well. Configuration steps are shown here:
If this isn’t clear enough, let us know here and I’ll spell out the steps.
Or if you want to learn more about the Mesh option, let me know and I’ll draw some pictures.