Sorry to say this on the Meraki forum, but this is really not a good application or strong suit for Meraki gear. That distance may work fine with a directional antenna, but you need to know that Meraki wireless gear does not do true bridges and can only extend a single VLAN. A pair of Engenius radios for around $150USD can do five miles and will last forever. I would save yourself some grief and money and use something like that for the bridge. Keep the Meraki for everything else where it shines in performance and management, etc. Consider the bridge like running a cable- you set it up once and should be able to forget about it.
- Ex community all-star (⌐⊙_⊙)