Huawei B525 connection issue

I am using a Huawei B525 in a mobile application (on a boat). When in a weak phone area (ie the sim does not get a strong signal), the performance of the whole router is degraded - this includes the performance hard-wired devices connected through the LAN sockets as well.
I have two NAS drives hard connected to this router, but access to these drives on a laptop is slow or non-existent when the sim signal is poor. Even trying to access the router setup page is painfully slow.
No problems at all with a strong sim signal. Is this a configuration issue? Any advice, please?

Most likely the processor is simply busy slowing down any traffic via the device.

It may be adding an Ethernet switch between router and all the Ethernet devices will improve performance - though won't help any wireless performance issues

Further to your suggestion, what if I were to use a separate wireless router for the NAS drives and the wifi distribution, and simply to use the B525 router (with the wifi switched off) purely as the internet connection (via its sim card), and feed this into the wan connection of the new wireless router. Would this then give me a "stable" wifi/lan network and would it then be unaffected by the intemittent performance of the internet connection?