Have you set the default gateway on the WAP to the IP address of your main router it is connected to?
DNS too - either to the other router (probably best, for accessing internal stuff in the other router) or to an Internet DNS service.
BT ADSL customer getting 1.7 Mbps (0.6 Mbps up) on a new road / new build development
CAB not FTTC enabled, not part of the 66% commercial plan. Not rural - no BDUK funding
(Virgin Media nearby)