SLT will put your router to Bridge Mode if you call and ask (Usually they do within the day). However with admin password also you can do the same (As mentioned above, the admin password is posted on Elakiri)
You can then enter credentials in the other router. However, connection slowness is likely due to WIFI strength. It would be cheaper for you to go with a good AP.
Can someone please tell me how this bridge mode or some other way to connect SLT ZTE F660 to a D-link AX1500 wifi 6 router? i just need SLT to use as a modem only. No wifi required