I fail to see the relevance of 192.168.1.252 ??
The IP address of the MT router on the ISP LAN, ( the WANIP of the MT Router) is 192.168.1.134
The IP address of the device on the ISP LAN wishing access to the pinter on the MT LAN is 192.168.1.116.
Even if you did some funky netmap ON THE MT router associating the printer IP 192.168.88.252 to the IP address 192.168.1.252,
How are you proposing the User on the ISP LAN reach the printer.
a. the dd-wrt router has no knowledge of the existence of the 192.168.88.0/24 subnet ??
b. the dd-wrt router has no knowledge of 192.168.1.252 on its network and it certainly would not send dst traffic for that address to 192.168.1.134??
The right approach I was thinking of was a simple static route on the dd-wrt router.
add dst-address=192.168.88.0/24 gateway=192.168.1.134
DONE!
Now the traffic will land at the MT router coming in on the WAN side.
Simple forward chain rule should sufffice.
add chain=forward action=accept comment=“incoming WAN to printer” in-interface-list=WAN src-address=192.168.1.114 dst-address=192.168.88.252
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Put another way, I do not believe the request is possible without some change of the config on the ISP router side.