Hi,
According to this link, the effective range of the Asus Zen Watch is about 5 metres (approx. 16') but it decreases if there are barriers nearby (usually large metal objects between the watch and the phone or electrical equipment which is emitting strong EMR). The link also explains how to troubleshoot pairing problems.
https://www.asus.com/support/faq/1009108
Here is a link to the user guide which explains how to pair your watch.
https://dlcdnimgs.asus.com/20160129_cosmo/cosmo/websites/global/products/r2eJwW68xEMKldzU/pdf/E-Manual_e9643_w1500qa_em.pdf