My friend has a minecraft realm, she can play on it with my wife just fine. When I join from home I get force kicked by host exception, as the game thinks our router IP is DDOS attacking her realm when I try to join. I can play on it just fine if my wife disconnects, or if I go to a friends house, though with covid that's not really viable. The only direct solution on my end would be to use a VPN to alter my IP. On the Coding end however you could lax the DDOS protection a bit to allow at least 3 people to join a realm from the same house. The only other alternative would be to copy her realms files, and just re-host her server myself. Which honestly seems like the fastest solution, especially given this has been a problem since at least 2018 from what I read in the forums already.
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This appears to be a duplicate of REALMS-3593, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.