Instead of assigning fixed IP on your router DHCP, you need to assign a fixed IP on Ubuntu configuration. As long as DHCP is in use, there’s an asynchronous delay.
Usually routers have a configurable DHCP address range. For example from .100 to .254 or something similar (C-class, /24 prefix). This leaves the range from .2 to .99 for fixed IP’s. So usually one would manually allocate one IP outside of the DHCP range for fixed IP devices. You of course can still assign a fixed IP from DHCP range and configure it as fixed IP on your server. But technically such would be more correctly “give me same address every time from DHCP spool” case.