Total posts : 45366
Dummy loads are most likely sealed against RF leakage but coax cables and equipment cabinets are usually not not. Leakage from the coax and the transmitter itself would explain the strong radiated signal you produced. From working on making a two meter system receiver enclosure RF tight I observed that VHF and UHF can leak like water and you literally have to make the enclosures watertight with metallic connections.
But this is incidental to your project. 433 MHz signals are not a good choice for building penetration and are easily attenuated by obstacles. Unfortunately, other choices such as 915 MHz, 2.4 GHz, and 5.8 GHz have similar issues.
From your previous description of the project it appears that the best and probably most workable common solution would be to use a stock FM band transmitter with a good gain antenna at the receive end. A gain antenna at the transmitter is not going to give any advantage over a simple whip since the legal limit is field strength at a distance and gain antennas function by increasing the field strength in a certain direction and the power to the antenna would have to be suitably reduced to keep the major lobe field strength within limits.
At the receiver end there are no regulatory restrictions on the antenna and the link gain can be achieved there. The problems will be mechanical but if these can be managed then a Part 15 link in the FM band has a good chance of working if you can find a “sweet spot” for the antenna. One trick I used for a VHF receiving system was to place the gain antenna (a Yagi type) above the suspended ceiling with the coax running above the ceiling to the receiver down the hall. It was out of sight and reach and worked well. You do need to be mindful of going through fire breaks with cables as it can be done but must be done properly.