skipping GF black box

After unsuccessful with ORBI RBK 852 wifi6 mesh router to connect my 2.4 Ghz devices, i bought RAX120.  Connected with GF (have 1 Gig plan) network box - no issues in connecting but the wifi speed on Galaxy 10+ - both with Ookla and fast.com is between 100 to 250 Mbps whereas from GF wifi the speed is 350+.  NG support tried to change channels but no improvement.  Later NG support said I should disable GF router function and make RAX120 as DHCP then only full potential of RAX120 can be utilized (!) - but stated they will not help in confugurng this.  GF support gave me mixed - one said they recommend always connecting the black box and showed my how to get to the setting but advised not to change and another said i can remove black box and connect directly fiber jack (FJ) to RAX120 (subject to my powering the fiber jack as their Box has PoE to power the jack but RAX120 does not have, i understand).  I connected  FJ to RAX120 - does not recognize internet - NG support tried with changing the MAC to my laptop (Surface Pro 6 connected through wifi) instead of ISP - no success.  Later they asked to try with wired connection between RAX120 and laptop - i do not have the adaptor - thought not convinced of this but i need to buy and test it.  The objective of mine is to connect more wifi devices to the router which GF box miserably fails.  in the meantime, any input on a possible solution and avoid double NATing.