This is a known issue on P99 (that isn't even exclusive to Linux); I believe it was introduced when Rogean had to strengthen the server's protections (after some asshole bitter player started DDOSing them awhile back).
Things you can try:
* switch your computer to use your phone's Internet (via a "hotspot" feature); even if this isn't a long-term fix, it will confirm that the problem is your network (if it works)
* get a VPN (they're only like $3/month, but I think some have free trials so you could check if they help without paying)
*
https://github.com/Zaela/p99-login-middlemand - this is an open source program a P99 player made which has helped a lot of people
I'm a Linux user myself, and P99 works without the above for me (now; at one point in the past I used to need the above) ... but I haven't installed it on my Steam Deck yet, so if the problem is specific to it I can't help.