There are multiple solutions possible:
1) Give the WAN port an IP from the same subnet (192.168.0.y), disable NAT on hAP, connect the WAN port (usually ether1) of hAP to the other router,
configure wlan so that it gives via DHCP IP addresses from the same subnet 192.168.0.z
2) Set the hAP into Bridge Mode, so that it uses IPs in the same subnet 192.168.0.z, configure wlan so that it gives via DHCP IP addresses from the same subnet 192.168.0.z
3) Don't use the WAN port, instead connect the hAP via a LAN port to the other router, use same subnet 192.168.0.x, configure wlan so that it gives via DHCP IP addresses from the same subnet 192.168.0.z
Ie. disable NAT on hAP and use the same subnet as the other router, and configure wlan so that it gives via DHCP IP addresses from the same subnet 192.168.0.z
Thanks for reply. Sounds like you are saying I can't split the functions so I have NAT on wlan1 (with 192.168.88.x) and flat repeater like function on wlan2 (with DHCP from either hAP or the base modem/router - I would not mind either but trust the hAP more)? I kind of assumed I could switch the whole device over but wanted to retain what I have as far as possible.
Another option I have considered is if I can persuade the audio unit (Bose) to attach to the garden network from the hAP. It might just
be in range. That would allow me to test the value and performance of the idea. It might also stop control of the music by a device in the house that switches away from the garden network - it is good being able to control the audio player from anywhere and any phone/tablet... But I can look into how well/badly that works. Note that music is not streamed from portable devices to the Bose unit - it streams direct from web radio/ Spotify etc the devices just tell it what I want (and save presets etc).