Customers - xx.xx.xx.70-128 /26 with xx.xx.xx.65 as Gateway
70-128 no can do + gateway should be in the range
WAN side of my Router - xx.xx.xx.66 /30
that /30 uses addresses 64 to 67 included (network and broadcast) so the next usable ranges are :
a /30 (68-71)
a /29 (72-79)
a /28 (80-95)
a /27 (96-127)
1 IP per range for the mikrotik and use this ip as gateway
1 IP is the network add. and 1 IP is the broadcast add.
it seems just strange to me that your ISP put his router on your IPs, i imagine you mean the router they installed at your location has the x.x.x.65 IP.
in that case i suggest do decrease the range on the ISP router at your place (a /30), a /30 on the MT also, then use RIP2 on your LAN so the router can learn the routes from the MT and vice versa
a drawing of your setup could help a lot