clean all surfaces with surgical spirit
and use a rubber solution glue,
maybe superglue, epo0xy etc..

or some bonding agent from a aquarists or boat supply shop..

biggest problem would be devisig a way to exert even tight pressure
on the joins while glue sets..

without permanently sticking the boots to your hands or anything else
they come in contact with..

i'm too skint to throw away good boots until the last
microns of leather upper have decomposed