The default address of OpenWrt is 192.168.1.1. It sounds like you're trying to connect from an upstream LAN (i.e. internet > main router > wan port of Xiaomi 4A R4C w/ OpenWrt; your computer connected to the main router with an address on the 192.168.137.0/24 network).
All unsolicited incoming connections from the WAN are blocked by the firewall by default. Therefore, you need to connect to one of your R4AC's lan ports, then try to connect via ssh: