On your UV editor, look on sidebar/view/display. Uncheck “pixel coordinates” (which are kinda useless for most purposes.) Now, select one of your bottom verts in the UV editor and look on sidebar/image/UV Vertex to see its position. I’m showing a V coordinate of 0.541. The default mirroring position is 0.5 (the center of the UV space). So select all UV coords and g y -0.41 to move the structure down to the midline of the UV space. After that, your mirrored UV will “connect”.
Alternately, you can set the mirror modifiers V Offset to 0.082 (0.541 - 0.5, times 2). Why times 2? Has to do with the transformation used by the mirror mod offset, which is designed to mirror about the upper margin at an offset of 1, or the bottom margin at an offset of -1. It is kind of confusing.