Friend on Same Router Cannot Join Cave

I am hosting a DST server, with three people:

1. Me
2. Housemate
3. Friend (not housemate)

We can play in the overworld just fine.

However, when trying to get into the cave, my housemate cannot enter the cave, as it gives "Server not responding" after trying to connect for several seconds.

Me and my other friend can enter the cave just fine.

I find that this is a very similar issue to this issue in 2018, below is my relevant server log:

[00:28:53]: [Shard] Begin migration #12 for (KU_7LPqa3A7)
[00:28:53]: [Shard] #12 Caves(624299474) -> [SHDMASTER](1)
[00:28:53]: [Shard] #12 -> session/5C856208A45A7F5D/A7GTAPQ90CK7/0000000047
[00:28:53]: [Shard] Received migration #12 data for (KU_7LPqa3A7)
[00:28:53]: [Shard] (KU_7LPqa3A7) disconnected from Caves(624299474)
[00:28:55]: New incoming connection 127.0.0.1|55389 <4899180213937757127>
[00:28:55]: Client connected from [LAN] 127.0.0.1|55389 <4899180213937757127>
[00:28:55]: ValidateGameSessionToken pgs-usc^KU_7LPqa3A7^DontStarveTogether^bnaK/6zZtCUquVoFSpQgx5lAUFQOaNVvJqd96YtFfRE= for <4899180213937757127>
[00:28:55]: Client authenticated: (KU_7LPqa3A7) username_me
[00:28:56]: [Steam] Authenticated host '76561198070860750'
[00:28:56]: [Shard] Completed incoming migration #12 for (KU_7LPqa3A7)
[00:28:56]: [Shard] Completed migration #12 for player (KU_7LPqa3A7)
[00:28:56]: [Shard] Writing save location file for (KU_7LPqa3A7)
[00:28:58]: [P2P] Received from 243.191.164.31|1 '76561198030334636'
[00:28:58]: [P2P] Sent to 243.191.164.31|1 '76561198030334636'
[00:28:58]: New incoming connection 243.191.164.31|1 <3933420895251624439>
[00:28:58]: Client connected from 243.191.164.31|1 <3933420895251624439>
[00:28:58]: ValidateGameSessionToken pgs-usc^KU_6uFGuHj5^DontStarveTogether^8AxfhutBY94ixZt6/EBWuuETMfF07l8BkaALzTJB12g= for <3933420895251624439>
[00:28:58]: Client authenticated: (KU_6uFGuHj5) username_housemate
[00:28:58]: [Steam] SendUserConnectAndAuthenticate for '76561198030334636'
[00:28:59]: [Steam] Authenticated client '76561198030334636'
[00:28:59]: There is no active event to validate against.
[00:28:59]: [Shard] Cancelling pending migration #11 for (KU_6uFGuHj5)
[00:28:59]: [Shard] Read save location file for (KU_6uFGuHj5)
[00:29:02]: Registering master server in Sing lobby
[00:29:03]: Resuming user: session/5C856208A45A7F5D/A7GTAPQ90CK7/0000000047
[00:29:03]: Could not find anim build FROMNUM
[00:29:03]: Could not find anim build FROMNUM
[00:29:03]: Could not find anim build FROMNUM
[00:29:03]: Player will spawn close to portal #6	
[00:29:03]: Spawning player at: [Load] (-290.24, 0.00, 13.00)	
[00:29:10]: Resuming user: session/5C856208A45A7F5D/A7GRGF8E26Q5/0000000047
[00:29:10]: Could not find anim build FROMNUM
[00:29:10]: Could not find anim build FROMNUM
[00:29:10]: Could not find anim build FROMNUM
[00:29:10]: Spawning player at: [Load] (-289.11, -0.00, 13.57)	
[00:29:12]: Registering master server in Sing lobby
[00:29:14]: Activating portal[6] to 624299474	
[00:29:15]: Serializing user: session/5C856208A45A7F5D/A7GRGF8E26Q5/0000000047
[00:29:15]: [Shard] Migration request: (KU_6uFGuHj5) to Caves(624299474)
[00:29:15]: [Shard] Begin migration #13 for (KU_6uFGuHj5)
[00:29:15]: [Shard] #13 [SHDMASTER](1) -> Caves(624299474)
[00:29:15]: [Shard] #13 <- session/5C856208A45A7F5D/A7GRGF8E26Q5/0000000047
[00:29:15]: CloseConnectionWithReason: ID_DST_SHARD_SILENT_DISCONNECT
[00:29:15]: Error decoding lua RPC sender
[00:29:15]: [Steam] SendUserDisconnect for '76561198030334636'
[00:29:15]: [Shard] (KU_6uFGuHj5) disconnected from [SHDMASTER](1)
[00:29:53]: [Shard] Begin migration #14 for (KU_U6kDdojc)
[00:29:53]: [Shard] #14 Caves(624299474) -> [SHDMASTER](1)
[00:29:53]: [Shard] #14 -> session/5C856208A45A7F5D/A7JODE6PUAR6/0000000047
[00:29:53]: [Shard] Received migration #14 data for (KU_U6kDdojc)
[00:29:53]: [Shard] (KU_U6kDdojc) disconnected from Caves(624299474)
[00:29:59]: [P2P] Received from 243.191.164.30|1 '76561198092420301'
[00:29:59]: [P2P] Sent to 243.191.164.30|1 '76561198092420301'
[00:29:59]: [P2P] Connection failed for '76561198030334636'