Why does the IP assignment in my router fail (only for my server, it works fine for everything else)

I just bought an old IBM X3650 M2 and installed Ubuntu Server 20.04 LTS. My router (Netgear R7000) is not obeying the IP rules I set in the router setup (and so far, I've only noticed this for this "new" server, not for the other 67 devices where I have previously assigned IPs in the router DHCP). In the IBM server configuration, I enabled the network (IPv4), and checked the "DHCP" option. My router assigns a random IP (192.168.0.5). I then assign an IP address in the router config (192.168.0.99) and apply. Once the router finishes restarting the DHCP, the server is now at 192.168.0.2. Rebooting the server (even power cycling) or re-assigning the IP and re-applying doesn't change anything. It seems the router is allowed to assign IP to the server, but the router won't obey its own rules. The rule is displayed loud and clear in the router config, it's not dissapearing (as I've seen other posts complain about). I know (think?) I could just assign a static IP in the server config, but I'd rather have it done by the router, for various reasons. And the question "why is it doing this?" is also bugging me :) Cheers! EDIT: I tried setting a static IP in the BIOS setup of the server. First time it worked (my testing IP, 192.168.0.99), but when I tried changing it to the permanent IP I need to use (192.168.0.100), it reverted back to the router appointed (.2) address. What more is, I can no longer set it back to the "testing IP" (.99). In the router, the change I made between being able to assign IP in BIOS setup and not, was to delete the entry for my old server (just an old NUC type computer, really), which was previously at 192.168.0.100.

Without seeing the router config it is going to be nearly impossible to answer, but my first guess is that the static assignment you made in your router does not point to the ubuntu server, and as such, the ubuntu server gets a fresh ip upon renewal. Doublecheck the MAC Address of the network port of your server and ensure that same mac address is in your router config. It should then work. You can also see on your router under leashes, which leash is static and which is dynamic.

That was also my first assumption; it seems like it dishes out dynamic IPs like there's no rule for the specific MAC. But the MAC is correct (the router automatically displays any devices not already assigned static IP in router DHCP when adding new rule, you just have to click the device you want to set a rule for). I even re-created the rule several times, and tried assigning a different IP a few times. I did also quadruple check that the MAC of the connected server (at 192.168.0.2) is the same as what I assigned IPs. They are identical.