After putting on metal roofing my cell signal increased.

The tin foil theory was incorrect ?.........
Are you sure you installed the metal correctly?

Metal roof will not affect cell service.
As for not using sheeting on a roof structure, but rather just 1x4s, you will have almost zero shear value, and for a stand-alone building, I would not stand under it.
I have done MANY metal roofs, and most were done with 30# felt as an underlayment.
I have also done a few dozen metal roof "overlays", going over 1 layer of shingles. I have done them both directly fastened through the roof shingles, (with a layer of 30# felt between the metal and shingles, for both condensation barrier and abrasion from shingle grit as the metal expands and contracts), and 1x4 skip-sheeting, being certain the 1x4s are secured to the truss or rafter under the original sheeting, not willie-nilly through the old sheeting. I used 24" centers for that, but for the life of the new metal on skip-sheeted roofs, anyone getting on that roof must be very careful walking on the rows of screws only, or the metal can and will buckle.

PF the 1x4 is only required if he already has inferior substrate down. If not and trusses are 4 or 8 foot o/c, would have to use a 2x4 flat or on edge depending on spacing 2’ o/c.