Netbsd wifi nic becomes inaccessible after some time

[bashrules]

I have an old Thinkpad with a wpi wifi nic running NetBSD 7.0. The networking system behaves strangly:

I have several computers on my wifi network. After an hour or so, these other computers can't access the thinkpad, anymore (ping, ssh ...). Only after I ping one of these other computers from my thinkpad, the pinged computer can access the thinkpad, again.

Whenever I'm ssh'ed into the thinkpad, the connection stays open for several days. After I close the connection, I can still access the thinkpad for an hour or so, but then it becomes inaccessible, again. Etc ad infinitum.

I don't think that has anything to do with the hardware, but rather with the software (NetBSD):

One of the other computers can have a ssh connection open to the thinkpad while another computer can't ping the thinkpad. Only after the thinkpad is pinging the latter computer, it can access the thinkpad, again.

I would like to access NetBSD anytime.